Oh! [ And then Strange realizes he hasn't explained the concept of billiards at all. Whoops. ]

Well, it's a game involving balls, played on a table with a pocket at each corner. You've got a long stick, that's a cue, and the goal is to use the cue to knock one ball into another ball, thereby knocking the second ball into one of the pockets. I don't think the Ringmaster actually has a billiards table in the carnival, but it wouldn't surprise me if the next place we stopped to visit had one that we could use.