A suppressor, often called a "silencer" is a device used to make a gun less loud. Suppressors are most often added to the gun barrel, usually by screwing the silencer onto the end of the barrel. Some guns have a built-in suppressor.
Even though many people use the name "silencer" when talking about a suppressor, it does not really make the gun silent. How much quieter it makes the gun depends both on the cartridges and the suppressor. A suppressed gun usually is between 120 to 135 decibels (dB). This is much quieter than without the suppressor, but is still loud enough to cause hearing loss without protection such as earplugs. An unsuppressed firearm usually produces 140 to 165 dB. For comparison, a typical jackhammer produces about 130 dB.
In most states in the United States, suppressors are legal for civilians to own, but the National Firearms Act requires that buyers must go through a special background check and pay a $200 tax. Owning an untaxed suppressor is illegal, except for licensed class 3 firearms dealers, the military, and law enforcement.