A silencer, also known as a suppressor, is an attachment put on the nozzle of a gun to mute or lower the sound of fire.
In Watch Dogs
In Watch Dogs it is not available as a separate item to purchase but is already pre-installed on certain weapons. Suppressed weapons allow one to kill enemies without others noticing (unless they witnessed the kill), which makes stealth much easier, or shoot them without directly alerting all enemies in the area. In-game, weapons with suppressors are known as Spec Ops weapons.
There are four silenced weapons in Watch Dogs:
Watch Dogs 2
In Watch Dogs 2, it will be available as a separate attachment.