That's a compensator. It helps the spent gases diffuse and dampens muzzle flip. However, it makes the pistol louder because the sound is channeled around instead of directly out the muzzle.
Chris, please head over to the Introduction forum and introduce yourself to the folks if you plan to stick around.
If ye love wealth better than liberty, the tranquility of servitude better than the animating contest of freedom, go home from us in peace. We ask not your counsels or arms. Crouch down and lick the hands which feed you. May your chains set lightly upon you, and may posterity forget that ye were our countrymen. ― Samuel Adams