From Fit Game Wiki
MET is an acronym for Metabolic Equivalent Value.
Metabolic Equivalent Values are multiples of resting metabolic rate. For example, Wii Fit boxing, with a Met value of 4.5 requires 4.5x more energy expenditure than someone sitting.
Unlike calories burned, METs are not dependent on your height, weight, age and sex. They allow you to compare different exercises for their relative value. Wii Fit Plus uses METs, together with your weight, age and sex, to work out the approximate number of calories you have burned during your exercise.