As per Indian Standard IS 456:2000, the Code of practice for Plain and Reinforced Concrete.
Letter M represents mix (concrete mix) and the number followed after M represents the compressive strength to be attained in N/sq.mm at 28 days age when a standard cube of 150x150x150 mm is subjected to standard compressive test. Both combinedly termed as Grade of Concrete.
Thus, a concrete of M30 grade should attain a compressive strength of 30 N/sq.mm at 28 days age.
Units/SI
N= Newton, for easy understanding 1 N= 100g considering gravity
mm= millimeter