A muzzle brake is a device attached to the muzzle of a gun that helps to reduce recoil.
Muzzle brakes work by redirecting the gases that are produced when a firearm is discharged, thereby reducing the amount of rearward force exerted on the gun. This provides benefits for both the shooter and the weapon, as it can help to make follow-up shots more accurate and increase the lifespan of the firearm by reducing wear and tear.
Muzzle brakes come in a variety of designs, but they all essentially work by channeling the flight path of hot gases exiting the barrel of firearms in a way that counteracts some of the rearward energy transfer to the shooter. Recoil mitigated in this way results in reduced muzzle rise, as well as less lateral and vertical movement of the muzzle after firing. Muzzle brakes can either be integral to the firearm (built-in) or they can be attached to the muzzle externally.
Muzzle brakes are commonly used on high-powered rifles where recoil can be a significant issue, particularly for those who are not accustomed to it.
Take a look at this exciting video about muzzle ports to see how it works: