Sweeping means the removal of debris from the pitch by means of sweeping with brooms and brushes.

Under the Laws the pitch must be swept clear of debris before it is rolled - to prevent any damage caused by debris being pushed into the surface by the action of the roller. The pitch should also be swept at every meal interval - the groundsman will usually ensure that the creases are re-marked then, too.
Sweeping won't take place if the umpires think that it might do something detrimental to the surface of the pitch.