This is a very informal masculine way of saying that you need to use the bathroom in Japanese.
Don't be surprised, though, if you're chatting along with a Japanese tourist, and he happens to tell you, "Excuse me for a second. I'm a toilet." In this sentence, boku means I, and toire means toilet, and wa is just the subject particle. So "boku wa toire" literally translates to "I am a toilet."