To pass up means to choose not to accept an opportunity or offer, even though it's available to you. When you pass something up, you're deciding to let it go by rather than taking advantage of it.

If your friend invites you to their birthday party but you pass up the invitation because you'd rather stay home and read, you're choosing to miss out. When a basketball player has a clear shot but passes up the opportunity to take it, they decide not to shoot even though they could have.

The phrase often carries a sense that the opportunity was worth considering. You wouldn't normally say you “passed up” a chance to do extra homework, but you might pass up a chance to try out for the school play, visit a new museum, or taste an unusual food. Sometimes people regret passing things up: “I shouldn't have passed up that chance to learn guitar when my uncle offered to teach me.”

The opposite of passing something up is seizing it, grabbing it, or jumping at the chance. When you pass something up, you're essentially watching it go by without reaching for it, even though you could have.