Movement (M) is a number used to determine creatures' speed in any given situation.
An average M for Humans is 4, which equates to 8 yards per 10 seconds moving at a cautious pace. A horse's M is 8, by comparison.
M is never used as the basis for a test - if the situation involves movement, use I instead.
Check the section on moving for full rules regarding the effects of Movement on the game.