Cant remember what M exactly stands for, but I know what it means. It is the ratio of your stack to the total of blinds/antes posted.
e.g.1 stack = 3000 chips, blinds 10/20, your M is 100
e.g.2 stack = 3000 chips, blinds 100/200 with 25 ante on 8 player table, your M is 6
What is the point of knowing your M?
the size of your M determines what strategy you employ. With an M of 100 you are a fully functioning poker player, with numerous options in many situations. With an M of less than 5 that Cod quotes from this tourney, you have 2 options. Push or fold
For M"s between 5 and 100, work it out for yourself, I know I haven"t yet!