Frivolous things are silly or unnecessary. If something is frivolous, then you don't need it.
Frivolous things are goofy, useless, or just plain dumb. The word is often used to describe lawsuits. A frivolous lawsuit has no value and will be a waste of the court's time, like someone suing McDonald's for making the coffee too hot (which actually happened). If someone is frivolous, that person shouldn't be taken seriously because he's always fooling around and never gets anything done. Frivolous is pretty much the opposite of essential.