Dear Navneet,
M20 and M25 is the Mix ratios of Cement, Sand and Crusher Stones (which can be used for construction).These are grade of cement concrete.
In M20 concrete mix the ratio of cement concrete is 1:1.5:3 i.e. 1 part of cement, 1.5 parts of course sand and 3 parts of bazri ( stone ballast 12mm to 20 mm).
In M25 concrete mix that the ratio of cement concrete is 1:1:2 i.e. 1 part of cement, 1 parts of course sand and 2 parts of bazri ( stone ballast 12mm to 20 mm).
Regards