To juggle is to toss several items in the air and keep them moving from hand to hand, with at least one of them suspended at any given moment. Most beginners learn to juggle using balls or beanbags, not knives.
A skilled juggler can juggle stuffed animals or oranges, or even knives and flaming torches. It's also possible to figuratively juggle, like when you juggle three jobs at once or juggle your responsibilities as a student, pet owner, and friend. In these examples, you manage several things at the same time. Juggle comes from the Old French jogler, "play tricks or sing songs," from the Latin root ioculari, "to jest."