Skill

Skills are the things that give monsters different moves. To make new skill sets, you combine them. Crack and Zam plus Crack and Zam equal Crack and Zam II, and Crack and Zam II plus Crack and Zam II equal Crack and Zam III. You can get many different skills that way.
You can also "level up" your skill sets by maxing the original one out. For example, if you put 50 skill points into Crack and Zam then when your monster is synthesized you can choose Crack and Zam II.