Some of the most visible liquidity sits at equal highs and equal lows: two or more swing points at almost exactly the same price. The more times a level has been tested and held, the more people feel safe putting stops just beyond it.
The sweep
A liquidity sweep is a move that briefly trades beyond such a level, then returns. The typical picture is a wick through the equal highs followed by a close back beneath them. The orders resting above were triggered; if that was the main reason for the move, there may be no further buying behind it and price can reverse.
Sweep or breakout?
The honest answer is that you cannot tell in the moment, only by what follows. A decisive candle that closes beyond the level and holds is more consistent with a genuine break of structure. A wick that closes back inside is more consistent with a sweep. Even then, a sweep may be followed by a second, stronger push.
What reversal evidence might look like
After a suspected sweep, some traders look for a change of character on a lower timeframe as evidence that the move has run out of steam. Treat that as one input among several, and size accordingly.
Context decides
A sweep that happens against a strong higher-timeframe trend is not the same as one at the end of a long, extended move. The same pattern means different things in different places.