Basically, an aircraft flies because of INDICATED air speed (IAS) which is the actual effect of the air on the wing similar to what you would feel if you were able to stick your hand out the window. If the best airspeed is 280 KIAS, then it will to a degree be the same at all altitudes.
But, the air is a lot thinner at altitude, so to achieve that 280 KIAS you will have to go FASTER through the air mass as you climb. At FL400, it would be about twice as fast so, that gives TAS, or true airspeed. For a constant IAS, the TAS will increase as you climb, and for a constant TAS, the IAS would be reducing.
The aircraft only cares about IAS...that's what makes it fly. TAS is what gets it from point A to B once corrected for the wind, and that gives GS or groundspeed.
Overriding all of that is mach number which is speed as a percentage of the speed of sound. It gives one set of maximum speeds and is related to temperature, and reduces as you climb. It's TAS. At sea level it will be around 680 knots, reducing to roughly 560 at FL400. So, that 560 kts of mach 1 is roughly 280 KIAS, which is not much faster than best IAS. Drag rises enormously around mach one, so it is limiting for a number of reasons. Above about .85, you'll slow to below the best IAS to keep away from it. In a climb, at about FL300 the climb IAS will hit roughly mach .8 and from that point you climb at a mach number.
An IAS, at a given weight, will give a fixed 1G angle of attack. An airframe has a 'best' angle of attack, so we use IAS to indirectly give us that best AoA.
But wait, there's more.................
as an aircraft burns fuel, and the weight reduces, it needs a lower angle of attack to produce the required lift. To maintain that best angle, the speed is reduced which is why cruise speed is constantly being reduced as a flight progresses.
And more........................
The engines have an RPM at which they function best too, sometimes it's 90% or even closer to full power. Reducing the speed means the engines may not be at that best power setting any more, so after a while you'll be better off climbing. The 'best' altitude would be the one at which the IAS giving that best angle of attack simultaneously requires the engines to be set at their best RPM.
The upshot of this is that you end up with a series of climbs and decelerations during any given flight.