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.
Bela Talbot attaches a suppressor to her gun when she sneaks into Sam and Dean's motel room to kill them, but she ends up shooting two inflatable dolls instead of the boys.
While sneaking into the Styne estate in Shreveport, Louisiana, Dean takes one of the bodyguard's guns which has been outfitted with a suppressor. Dean uses it to kill two guards and enters the estate, but he is subdued and has both of his guns taken from him.
Ketch has a silencer attached to his gun when he shoots and kills Mick Davies.