Sweeping means to send out a single party member ahead of the party to scout and trigger off any traps or surprises in the area ahead, much like a real-life Mine sweeper.
Sweeping is useful when exploring an area for the first time as well in some high-level areas where enemy pop-ups are a frequent phenomenon such as Urgoz's Warren and the Tomb of the Primeval Kings.
The Sweeper should be familiar with the area he/she is sweeping and should also have strong survival skills. A sweeper who dies often and accumulates Death Penalty is not very useful as a sweeper.