Frivolity is a type of clownishness or silliness. Frivolity is the opposite of getting down to business in a serious way.
If people are running around a classroom, throwing things, and laughing, the teacher might say, "Why all the frivolity?" This is a word for clowning or horsing around — not being serious. There's a happy flavor to this word. Giggling when you should be listening could be considered frivolity. Also, anything that's insignificant is a type of frivolity, as in "That's nothing! Just a bit of frivolity!"