Henry,
High speed and low speed buffet are terms which simply refer to the airplane aerodynamic response to certain flight conditions. A low speed buffet is nothing more than a stall buffet; it's based on angle of attack and in it's simplest terms, just like any airplane in training, get it slow enough and it will stall. Bank it steep enough and it will stall. It's just a stall buffet.
As an airplane climbs higher and higher at a constant indicated airspeed, true airspeed increases...the airplane goes faster and faster. This is due to air density. Again, on the most simple level, where there's less air resistance the atmosphere is thinner), the airplane goes faster over the ground in still air while indicating the same airspeed in the cockpit. The airplane must move faster through thinner air to produce the same lift. The airplane must move faster to produce the same pressure to produce the same indication in the pitot tubes which tell the airspeed indicator how fast the airplane is going.
Conversely, if the airplane goes faster and faster (higher true airspeed) for the same indicated airspeed, it also holds true that the airplane flies at a slower and slower indicated airspeed for the same true airspeed (other airspeed descriptions fit better, but become more complicated..such as equivilent airspeed, etc...it's easier right now to think in terms of indicated, and true airspeeds showing how fast the instrument says you're going, and how fast you really are). If you hold a given true airspeed and keep climbing, you'll be flying at a lower and lower number on the airspeed indicator as you climb.
While that relationship is going on, another is important. The upper limiting speed of the airplane is the maximum operating speed, known as Vmo at lower altitudes, and Mmo (mach limiting airspeed) at higher altitudes. On the airspeed indicator Mmo is shown as a barber pole (above which on a tape is the "red army" to which you're referring, though there are a number of ways to display information, ranging from angle of attack indications to tapes to round dials...I'm going to use round dials to keep it simple). As you climb higher and higher the mach limit of the airplane decreases. The barber pole striped needle on the indicator may start out at say, 400 knots indicated airspeed at sea level, but at cruising altitude may indicate a much lower number, such as 230 knots.
At altitude, we cruise in relation to mach airspeed rather than knots indicated airspeed. Mach is simply a measurement of the speed of sound in a given air density...that speed decreases as we climb. Much like indicated airspeed. For our purposes, we really don't care how fast sound goes, but it's a good reference number for us because it measures certain functions of air density...most important for us, mach effects...or in other words, what the atmosphere does for us and doesn't do for us at high altitudes and high airspeeds in relationship to our reference speed, the speed of sound.
Hopefully I'm not confusing you. We cruise at .84 Mach all the time. In our airplane, it's the number we use pretty much everywhere at altitude. .84 mach, or in other words 84% of the speed of sound at any given air density, occurs at a much higher indicated airspeed down low than it does up at cruising altitude. .84 is well below any high speed aerodynamic limitations, and keeps us fast enough that we don't stall while flying level or when turning (remember that your indicated stall speed goes up in the turn, so we want to cruise with a high enough airspeed that we can make a standard turn at altitude and still be comfortably above stall margins.
As we climb higher, the stall speed increases. Air is less dense, the airplane must fly at a higher true airspeed, but lower indicated airspeed, to make the same lift, and at a higher angle of attack. A low speed stall or buffet is a function of angle of attack; at some point you reach an angle of attack, the angle at which the wind meets the wing, when the wing will stall...every bit as much in an airliner as on a paper airplane. The speed at which that occurs straight and level will depend on several factors, (weight, altitude, temperature, bank angle, configuration and power setting, among other things), but represents the lower buffet limit. Just think of it as don't-get-too-slow-or-you'll-stall. That speed gets higher and higher as we climb. For a given weight, the stall margins, or space between the airspeed/mach number at which we're cruising now, will shrink with an increase in altitude. We might have 50 knots to play with here at this altitude, for example, but if we climb higher there might be a 25 knot or 10 knot difference between our cruising airspeed and the low speed buffet or stall. This plays part of the role in calculating our cruising altitude, and the altitudes of which we are capable at any given time based on some of the things we've discussed.
The high speed buffet is a little different. That's a result of what are called "mach effects," or simply put, the effects of going too fast. At lower altitude in a light airplane, we don't deal with the effects of air compressibility...so we don't talk about it. We don't consider low altitude subsonic air to be compressible. At higher altitudes at higher mach numbers, especially transonic numbers between about .75 and 1.2 mach, the way the airflow around the airplane affects us changes dramatically. It's the reason we have swept wings and flying surfaces, and the reason we measure our speed in relation to mach (percentage of the speed of sound); it's a measurement of our relationship to actual mach, which is a measurement of how the air is bunching up and compressing around us.
Air compresses to form shock waves and bow waves which change the way pressure is distriubted in front of, across, and behind the airplane. It changes the way the airplane displaces air, and displaced air is how we create lift. Lift is nothing more than a change in pressure around different parts of the airplane, and can be thought of as more pressure underneath than on top. As we move into higher mach numbers, pressure also builds in front of the airplane and forms a wave of compressed air. This wave moves aft and across the wing, and the properties of that air and the way the wing affects it, and the way it affects the wing, change depending on where it is.
Among those effects are things you've probably read about, such as mach tuck, and of course, high speed buffet. You can think of high speed buffet as a warning that we're going too fast. Just like a low speed buffet being produced by air burbling around the wing and striking the wing, tail and other parts of the airplane in a turbulent, random fashion, a high speed buffet is the airplane starting to react to airflow in a different manner. The drag rises sharply, lift can decrease, control effectiveness can be reduced, the airplane may experience a loss of downward force on the horizontal stabilizer or a change in the center of lift on the wing as the shock wave moves aft across it. It may want to speed up even more or begin losing elevator authority or reach it's trim limits if one goes too far. It may shake, or buffet, or it may display nothing really significant, depending on the airplane, it's weight, etc.
We're given upper and lower buffet limits as an operating range in which we must stay. It provides margins in which we can safely operate without going so fast we're into the mach effects, and without going so slow we stall. Sometimes the buffet margins are referred to as stall margins; a high speed tall and a low speed stall. You can also think of the upper buffet margin as a high speed stall in that the airflow changes about the wing (in some cases drastically) to where aircraft control may be difficult or impossible in some airplanes (if flown fast enough), lift is reduced, control forces may become too light (or in some cases too heavy), etc...and the drag rise is so high it saps all the efficiency out of the flight. You think of a low speed stall as a buffet because of some of the same reasons; the changes in pressure about the wing and airplane change drastically, lift is lost, control is affected, etc. Both can produce a buffet (or in some aircraft no buffet, but other warning devices are installed to replicate the buffet for the pilots in the cockpit).
Operationally, at cruise altitude, data is available in form of buffet margins. I use an old fashioned round dial airspeed indicator. I set one of the little plastic bugs in the window of the airspeed indicator to show me where the low speed buffet margin is at cruise, based on data available to us in the cockpit. I use the barber pole or mach limit as my upper margin...though actual buffeting may take place a little before or after that point.
The higher I climb, the closer those numbers are together. You may have heard the term "coffin corner," which is a place where those two numbers come together, or very close together. You can't slow down because you'll stall, and you can't speed up because of mach effects. You're trapped in a very narrow operating range or speeds. Some aircraft like the U-2 operate at such high altitudes that they literally spend most of their flight in the coffin corner...with only a few knots either way before they're either into mach effects of going too fast, or stalling for going too slow. Hopefully that helps.