Imagine you have a person, this person would just use some shorts, shoes and empty handed.
Now, you want the same person but wearing an armor, and a sword on his hand, but you want to use the same skeleton and animations.
With skins you can do that, you just change the skin attachment for a slot (just asign another image as usual), and then you can have several versions for the same skeleton/animation, without having to redo these two things.
Hope it is clear.