A muzzle brake is typically a device that is attached to the muzzle of a rifle. Its purpose is to reduce the recoil of the rifle when it is fired, allowing the shooter to keep the sights on target for longer, and thereby achieve greater accuracy.
There are a variety of different muzzle brake designs, but they all work by redirecting some of the gases that are expelled when the rifle is fired. This redirected gas can help to counterbalance some of the recoil force, making the rifle more stable and easier to control.
Muzzle brakes are also pretty handy in reducing the amount of discharge smoke from the gun. This smoke prevents you from seeing your target after firing or seeing the spot where the shell lands to help you correct your aim.
This means they provide two crucial benefits in one device.