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.
Gordon uses a sniper rifle loaded with subsonic .223 caliber rounds and outfitted with a suppressor to try and kill Sam in his motel room. Dean stops him before he can succeed, but ends up getting knocked out by the rifle and captured by Gordon.
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 outfits his gun 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.