Obvious levels such as a prior day high, a double bottom, or a round number collect stop orders. When price trades through them, those stops become market orders, which provides the volume that a large participant needs in order to fill a position in the opposite direction.
On the chart a sweep looks like a spike with a long wick beyond the level, immediate rejection, and a close back inside. It is the mechanical explanation behind classic terms like stop-hunt, bull-trap and fakeout.
Two cautions. First, it is unfalsifiable in the moment: every failed breakout can be called a sweep afterwards, which makes the idea feel more predictive than it is. Second, a break that keeps going is simply a breakout. Only price returning inside the range distinguishes the two, and that takes time you may not have.
Related: equal-highs, false-breakout, stop-hunt, change-of-character