M is the number of orbits you could fold every hand and still be sitting there. It is the only stack measurement that accounts for the thing actually killing you, which is the blinds going up whether you play or not.
Enter the stack and the level. The zone tells you how much of a game you still have.
Twenty big blinds means one thing at a table with no ante and something much more urgent at a table paying nine of them every orbit. M counts everything you are forced to put in, which is what actually sets the clock on your tournament.
At a five-handed table the blinds reach you twice as often as at a full one, so the same stack is under twice the pressure. Effective M scales for that, and it is the number to use once a final table starts losing players.
They describe how much choice you have left, not what to do with it. A green M means you can wait; it does not mean waiting is right. A red M means you cannot, and that part is not negotiable.
Above 20 you have a full game. Below 10 your options narrow quickly, and below 6 you are looking for a hand to move in with.
Big blinds ignore antes. M counts the whole orbit, which at a table with antes can be half as much again.
Paul Magriel originally, popularised by Dan Harrington in Harrington on Hold'em, which is where the colour zones come from.