M is your stack divided by the total of the blinds and antes for one round of hands. Assume the blinds are 100/200 with a 50 ante and there are 10 players and also that your stack is 8000, then M = 8000/800 = 10.

It comes from Harrington on Hold'em, more specifically volume 2 and is used for deciding when to open push in tournaments, among other things.