A sweep is a reversal executed from guard: you use your legs and leverage to unbalance the top player and come up on top yourself. Sweeps score two points in IBJJF competition and represent the primary offensive path when submissions are not available.
Good sweeps attack the opponent's base. The scissor sweep attacks a narrow, upright base from the side; the pendulum sweep attacks when the opponent's weight is too far back; the hip bump attacks a passive, upright opponent. Every sweep has a weight distribution it targets.
A sweep attempt that is blocked is often a setup for a submission, and vice versa. This threat pairing is the core of guard offense at every level.