Uhh
A skin is basically a piece of sticky paper you put on something.
In this case, it's a skin for a computer. It's a sticky paper you put on your computer for an alternate design than what's given to it.
It doesn't add bulk since it's literally a thin piece of paper, but it's just there for design purposes. This specific one is also supposed to protect from scratches and stuff.
Where a case for something is supposed to be both aesthetic and protective, a skin is just purely aesthetic.
And I don't know when it's coming in, maybe a week or so from Tuesday?