A trifle is something that's totally unimportant. If your friend is freaking out over which poster to buy and you call her dilemma a trifle, you're saying she shouldn't get so worked up over nothing.
Trifle comes from the Middle English trufle, meaning "fraud, joke, trick." You'll usually hear it used to mean "insignificant, trivial." If you write a little ditty to sing at a birthday party, you could say it's just a trifle. As a verb, trifle means "toy with, waste." If you play with someone's feelings without seriously meaning anything, you are trifling with his affection. Trifle is also a sweet, sticky cake. Don't trifle the day away dreaming about trifle. Make some!