M is the ratio of a player's stack against the size of the blinds and antes combined. If the blinds/antes add up to 700 and your stack is 3500 then your M is 5.

Also related is Q, which is the ratio of a player's stack to the average stack. In general, you should regard M over Q, in other words, if your M is high but your Q low (for example if someone has knocked out 6 people in the first level) then you might have a low Q but a high M, and thus no reason to sweat.