A muzzle brake is commonly used on high-powered firearms to reduce the recoil felt by the shooter.
The basic principle behind a muzzle brake is that it vents gases from the barrel to help counter recoil. This allows the gun to redirect some of the energy that would normally be going backward into shooting the bullet forwards.
Muzzle brakes come in all shapes and sizes, but most work by channeling the gases exiting the barrel through a series of passages or ports cut into the brake. These ports can be oriented in different ways to achieve different results, but they all work towards redirecting some of that gas away from the direction of rearward travel. Some muzzle brakes also feature escape ports near the muzzle end of the firearm which serve to disperse some of the gases to the sides, further reducing recoil.