Slice, or backspin, causes the tennis ball to spin back toward the source of impact (you). When a slice shot bounces, the ball stays low, forcing your opponent to really stretch to get to the ball. If players like to attack the net and volley, they slice a lot.
The slice can be a useful weapon, but it does have some disadvantages. Slice shots never travel with a lot of pace. Slice shots that are hit from lack of confidence or in a desperate attempt to just get the ball back tend to float, and they travel through the air slowly. Your opponent can do almost anything he wants with them, especially from a position up at the net.