Coffin Corner
Redout, to answer you question simply, Think of an inverted "V" or an "A" without the horizontal line. You will notice that the two legs are far apart at the bottom and converge toward the top. The bottom part is sea level and the top (where the lines meet) is your high altitude limit. So at sea level there is a wide space between max speed and stall speed. However as the aircraft climbs, the two lines get narrower which means the difference between max and min speeds is getting less. Eventually you get to an altitude where if you increase or decrease your speed the aircraft can no longer maintain lift. It will either stall or fall. So, how to get out of that situation? Know the performance of the 'plane, know the max safe altitude for a given weight and don't go there. I think you answered the question yourself, you must descend to a lower altitude to increase the margin of safety. Unfortunately turbulence at altitude can cause speed fluctuations which can cause an upset to occur.
Hope that helps.
T