" ‘M’ in it’s simplest form is simply the ratio of the blinds (and antes if applicable) to your current chip stack. At a full table you add the 2 blinds together and divide your chips by this number to give you an ‘M’ number. For example if you have 3000 chips and the total blinds are equal to 600 then your ‘M’ is 5.
What this number tells
you is how many more ‘rounds’ of the table you can survive without being blinded away. In the above example, assuming you played no hands, you can last for 5 rounds.
The basic ‘M’ score assumes a full table of 9 or 10 players. When there are less players the blinds will come around faster – and so your M is adjusted to compensate. Dividing your current M by the ratio of players at the table is know as ‘Effective M’. For example, you have 5000 chips with 5 players left – the blinds total 500, your M now equals 5000/500 = 10, however only 50% of a full table are playing so you divide by 2 for an effective M of 5."
I took this right off of an ebook hope it helps!