I first read M in Harrington on Hold'em. It is refering to you chip stack in relatation to the blinds and antes in a tournament. For instance 1500

chips with 10/20 blinds gives you an me of 1500/30=50 so you would say your M is 50.

When there are antes you have to count the players at the table with you so if your stack is 50K with 1000/2000 and 100 ante at a 6 person table the calculation becomes 50,000/(3000+600) or 13.89.

M is used to determine strategies for stages of the tournaments and effects the way you should be playing your hands.

I hope that isn't too confusing.