To "horse around" is to play roughly, without regard for normal limitations such as rules or safety.
Real horses play very energetically and roughly. This is why "horsing around" creates an image of rough, physical activity. However, children "horsing around" can cause damage to private property, or injury to themselves or each other. Parents and school teachers usually limit or ban horsing around for this reason.
Example: "Ted and Tommy were horsing around outside. Because of that, Ted tripped and scraped his left knee. Their mother should tell them to tone it down."
The word horseplay (a noun) represents the act of horsing around.