M is the ratio of your stack to the total blinds and antes per orbit.
Lets say your stack is 1500
SB = 100, BB=200, Ante=25, 8 players
each orbit you have to pay 100+200+8*25 = 500.
Your M = 1500/500 = 3

It means how many orbits you will have until you will be blinded out if you dont play any hand.

When you have low M your arsenal of moves dwindle (for example with high M it is OK to limp SC from LP to see a flop, with M of about 7 it is not a good move)
Eventually with low M you have exactly one move - go AI preflop.

HoH2 (HoH is an abbreviation of 'Harrington on HOldem' ) has a long discussion on M and his zone theory, it is highly recommended reading