When you hit the public restroom, prior to a long engagement (such as a concert, ballgame, or plane ride), to avoid painful need to urinate, many hours into the event
Be right there, playa! Gonna pre-pee before the big meeting, yo!
Primarily for movie goers, the pre-pee is when one goes to the bathroom before getting the movie starts, in order to avoid having to go to the bathroom in the middle of a movie and miss a good part.