Hey guys, I've always heard people say things along the lines of "this guitar has a really nice satin finish" but as much as I try to get to know it better, I can't seem to be able to do that. Maybe one of you guys can help and can you tell me if it's done on bodies too, or just the necks, thanks
It's done on bodies and necks.

In paints you normally have "Flat" "Satin or Semi-Gloss" and "Gloss"

Flat is no shine, satin is half shine, and gloss is full shine. By shine I mean you can see reflections in it.

Satin body paint on an ESP Eclipse:

Notice how it sort of reflects light.
If you go to home depot (or any paint store) you'll find a whole range of paints that range in gloss level from very glossy to flat/matte, with a lot of steps in between (eggshell, satin, semigloss, etc.) Essentially, paints start out glossy and then have deglossing agents mixed in for the other looks.

Satin and matte finishes on guitars and cars are sort of the fashion now. At some point in the future they'll look as modern as vinyl tops and red-line tires and fins on cars do now.