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.

Actually that's not the best use case that Cuellarjmcg is giving. Skins should generally not be used for something like changing weapons, it's better to have multiple images on a slot for that. What it can be used for though is larger changes like what is done with the goblins example.
If you want to mix up a ton of different items a character can wear you won't be able to do that using skins, since you can't have multiple skins active at the same time.

Ah yes, good point. Thanks for the correction. The way I see them is to create different actors from the same skeleleton, but yeah just changing slots is better for these cases

To have characters wearing different items, just use images on a slot. Use skins when you want to reuse a skeleton with animations that change what images are visible during the animation. These image changes in the animation can hide and show a skin attachment instead of an image. The actual image that is hidden and shown comes from a skin, allowing the animation to be reused.