Wetsuit thickness is measured in millimeters and the first number measures the center of the suit (your core or sternum) or torso and the second number is the thickness of the material around your appendages (arms and legs). So in a 3/2MM wetsuit, the torso of the wetsuit is 3 millimeters thick and the arms and legs are 2 millimeters thick.  The thicker the suit the more warmth it provides but the tradeoff is less flexibility.