The slice serve lets you hit a very short cross-court ball at 0-0 with a trajectory that goes extremely wide. It's used to draw the receiver outside the court.
When done properly, it opens up space for the server, who can then play the ball into the open court. This tactic is very simple and it takes just a few strokes to score a point.
Of course, the receiver has some tricks up their sleeve too. If they can see a slice serve coming, and it hasn't been performed well, they can send a devastating return straight down the line or cross-court.
You therefore need to be very careful when setting up your serves.