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, many times by screwing the silencer onto the end of the barrel. However, 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.. Most guns are still very loud even with a suppressor added. It is important to wear ear protection even with a 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.