It'll be DLC, so you need the current version of the game to play it.
An yeah, just like dutchvanya said, a season pass usually means you also get all the upcoming DLC for the game too.
10 bucks to support a cool developer and to get an interesting looking game on top of that seems pretty cool to me. Still, as always with pre-ordering and backing projects, you're buying a blind bag which could be good or bad.