MRC stands for Medical Research Council which is the Institution that came up with the Muscle Strength Scale.
The different categories are as follows:
Grade 0: No muscle activation/movement
Grade 1: Muscle activation with trace/flicker of movement
Grade 2: Movement over full ROM if gravity is eliminated
Grade 3: Movement through full ROM against gravity without resistance
Grade 4: Movement through full ROM against gravity + moderate resistance
Grade 5: Movement through full ROM against gravity +full resistance
N.B. Grade 4 and Grade 5 can only be distinguished if both limbs are compared!