This may sound noobish, because it probably is. But what exactly does Mecanim DO? Can you create animations with it? Import animations? What exactly?
Mecanim is Unity’s new animation engine (the old one is still available if you need it). It allows you to import your already made animation clips to reuse and re-purpose them. Right now, it only really works for humanoid models, but that will be improved in the future.
With Mecanim you can import an idle, walk, and run animation, then easily transition between the three based on the character’s speed (all with no, or very minimal, coding). Once you’re done with that, you can apply the same animation scheme to any other humanoid model and have them do the same thing without having to redo it all.