What is M2?
What is the meaning of M2? and M3?
Some context would help us here, but I can think of two uses:
1] They are indicators of money supply - you can look here for a definition: Money Supply definition
2] They could be designations for British Motorways
Have you got an example in mind?
If you're a gun nut, then they are a designation of two types of rifles.
Where did you see it?