Coffin Corner
As altitude is gained, the air thins. Try breathing on top of Mt Everest.
As the air thins, the speed at which an airplane will stall INCREASES. At the same time, the speed at which an airplane enters the supersonic realm DECREASES.
Imagine a vertical bar chart. Up, down, right? Go ahead and draw one on a piece of paper right now. It will be a lot easier if you have a picture. The drawing does not need to be pretty.
Now graduate the vertical bar you've drawn with horizontal marks. These marks, or ticks, are the airspeed. The unit of measurement is the knot.
At the top of the bar chart, color the ticks in red. Make these ticks, or graduations if you like, thicker than the others. The red ticks represent a high speed "no-go" zone for the airplane. Flying in the high speed red bits at the top of the bar chart reduces overspeed protection. The airplane is now approaching the supersonic realm.
At the bottom of the bar chart, color the ticks in red. Make them thick, just like the red ticks at the top of the bar chart. The red stuff down here is the low speed stall area. Fly in it, that is, reduce your airspeed enough, and your airplane will fall out of the sky.
Now, as your airplane climbs ever higher into thinner air, the red at the top of the bar chart comes down. You don't have to draw this. The flight instruments in a real airplane show the red ticks coming down. At the same time, the red at the bottom of the bar chart rises.
At some point the red bits will be so close that the airplane cannot slow down by a knot, nor can it increase by a knot. Doing so would see the airplane stall on the one hand, or overspeed on the other. The airplane now has no 'manoeuvre margin'. None at all. This is what we refer to as 'coffin corner'.
No pilot will knowingly fly his airplane (masculine encompassing the feminine) in 'coffin corner'.
In another type of graph used to depict reducing manoeuvre margins for flight due to increasing altitude, the presentation is of an upside down V-shape. Each side of the upside down V-shape represents airspeed. On the left is the low airspeed stall. On the right is the overspeed 'speed'.
At low altitudes, the upside down V-shape is wide. There is a wide manoeuvre margin. At high altitudes, the upside down V-shape is thin. Here, there is a thin manouevre margin. Coffin corner is at the top of the upside down V-shape.
I hope that helps.