Sure, it's kind of an old-fashioned form of entertainment. The Victorians were really into it, it went out of fashion around the turn of the century when people started to get more concerned about the treatment of animals and I guess you can't beat the special effects of the big screens. But it regained some interest in recent years.
So, pretty much it's a sort of live performance where you can see stunts, tricks, death-defying acts, illusions. Basically what the movies can do except it's real. People like real risk I guess. There are also fairground rides and games and a side show (what they used to call a freak show) where you pay to basically look at people who are kind of... unusual in some way.
This is probably a longer answer than you were hoping for.