Okay kiddo, a taser is a special kind of gun that the police use to stop bad guys or dangerous people. They don't use real bullets, but instead they shoot out two small dart-like things that have wires attached to them. These darts stick onto the person and the wires send an electric shock to the person's body. It's kind of like getting a zappy feeling like when you touch a door knob after scooting your feet on a carpeted floor. This zap makes the person's muscles go all floppy and they fall down to the ground.
The taser gun has a special battery in it that powers it up, and it can only be used on people who are old enough and big enough to handle the shock. When the police use a taser, they do it to keep everyone safe and make sure that nobody gets hurt.