I just bought the game and it is in the Into the Abyss box, but it is supposed to contain the "base set". What does that mean? Can someone help me out? I'm new to the game.
Not sure what they told you, but the Into the Abyss expansion is not stand-alone.
While into the abyss has the largest card variety of the expansions, you need the base cards: regulars, torches, etc.
you need to get those from somewhere.
There is a decent chance you can get those second had since many thunderstone fans have ALL the sets they could find, and therefore have more then 1 basic card set.