Popularly known as a silencer, it's a device attached to a firearm to reduce the amount of noise and flash generated by firing the weapon. It generally takes the form of a cylindrically-shaped metallic tube that is fitted onto the barrel of the firearm, and sometimes reduces the velocity of the bullet.
Sam: These are .223 Caliber, subsonic rounds, the guy must've put a suppressor on the rifle.
Ava: Dude, who are you?
Sam: I ah...I just I...I just watch a lot of TJ Hooker.