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 the sound is reduced depends both on the cartridges and the suppressor.
In most states in the United States, suppressors are legal for civilians to own, but the National Firearms Act requires that buyers must undergo an extensive background check and pay a $200 tax. Possession of an untaxed suppressor is illegal.