To brandish something is to wave it about aggressively, as one might brandish a sword or tennis racket (if it's a particularly intense game).
Brandish often implies that a person is wielding a physical weapon. In fact, brandish comes from the archaic French word brandir, meaning “sword.” However, it’s also possible to brandish objects that aren’t weapons. If you win a bowling tournament, for example, you might be inclined to brandish your shiny new trophy in front of the other competitors. Be careful, though; they might mistake your trophy for a "brandir" and respond accordingly.