Someone posted the "real" meaning once, it was pretty sickening. Something about drinking...out of a skul, or something like that.
Someone will know the EXACT meaning.
The best explanation I've heard (and posted once, I think) was this:
In Valhalla when the Vikings feast, they drink out of the skulls of their defeated opponents, and when they drink they'd yell "Skul!", meaning use your opponent's decapitated skull to drink, and everyone would drink. So for all intents and purposes it means "raise your glass" or "cheers!" So skul turned to skol, so now it means "cheers" vikings. So basically when we say "Skol Vikings" it means a salute or a toast to the Vikes.