Peachey explains it well. LOL at the example. :) To further add, it is meant as a compliment. First impressions can be misleading and you might find out a surprising quality about someone. ex- You find out the star athlete of your school plays piano and loves studying math. You could definitely say, "there is more to him than meets the eye."

The things which "meet the eye" are simply the things you see, or the first obvious impressions. When "there's more than meets the eye", it means there are hidden qualities which are not obvious at first glance. Like a truck which actually transforms into a giant robot.