basically, exactly what it is called. hugging a person while either you, or you and the other person is sweaty. its a good thing, considering most people consider sweat to be gross ( which I dont, unless its on a nasty, unhygenic person ) ..so if someone gives you a sweat hug, it means they like you, enough to get your sweat all over them. O.O yep.
Girl, all sweaty after dancing like a nut to a guy: SWEAT HUG! ( they proceed to hug...sweatily... )
a bunch of friends randomly sweaty: SWEAT HUUUG!!! **they all hug**
...see? very simple.