It is a muzzle brake with a slant cut into the end. Muzzle brakes are used to reduce recoil and in hand reduce muzzle climb.
Slant brakes are pretty bad, I'd look for another option if you can. They just don't control climb and flash like birdcage or vortex style brakes do.
Slant brakes are primarily used to give the AK-47 an "authentic" look, plus they are the only muzzle device that permits the mounting of a bayonet on an AK-47. None of the preceding is true for AK-74 rifles. I have heard good things about this muzzle device:
