The phrase "to clown around" means to behave in a playful, exaggerated and boisterous manner that is typically associated with clowns. It can refer to someone who is acting silly or goofy for fun, often by making faces, playing practical jokes, or engaging in other forms of lighthearted tomfoolery. The phrase can also be used more broadly to describe anyone who is being overly dramatic, flamboyant or theatrical in their behavior.