Whats an overlay?

I bought some kits last night & some of them have overlays..
Any examples on what those are?

I'm not sure exactly. How big is it? Like, would it fit over a 12x12 page, or is it small? I use them behind siggies and such to have a background, but still have it sheer. (If that makes sense? Look at my profile, I have one of dd that has an overlay type background) or you can layer them on top of backgrounds to change the look.