Horseplay is a rough, rowdy kind of fun. If you and your friends are jokingly pushing each other into a swimming pool, it's horseplay.
Horseplay is vaguely dangerous — someone could get hurt, and that's why you might hear a teacher, babysitter, or parent yelling, "Hey, kids! Knock off the horseplay!" The origin of this word is a bit vague, although it's known to date from the 1500s, probably from the sense of a horse as a heavy, strong, rough animal.