A muzzle brake is a device attached to the muzzle of a firearm to redirect propellant gasses upward, which reduces recoil and allows the shooter to keep the sights of the firearm on target for longer periods of time.
Recoil occurs as a result of the gunpowder being ignited in the cartridge. The expanding gas creates a large amount of pressure that propels the bullet down the barrel. This pressure also pushes backward on the gun, causing it to recoil.
Muzzle brakes are designed to divert some of this gas upwards or sidewards, which reduces the backward force on the gun, leading to less recoil.
Muzzle brakes are most commonly found on rifles, but they can also be used on shotguns and pistols. They are not effective with short-barreled firearms.
Muzzle brakes can be very effective at reducing recoil, but they also tend to increase the amount of noise and muzzle flash that is produced when firing the weapon. For this reason, they are not always popular with shooters who are concerned about these factors.
Watch this video and see the difference: