Silencer with standard 14mm Counter-Clockwise / Positive threading to screw strait on to thread compatible barrels. AEG silencers do not actually make Airsoft weapons quieter though they can change the nature of the sound, particularly with gas weapons and can subtley muffle spring rifles.
Besides aesthetics, silencers can also be used to conceal longer inner barrels so whilst a particular AEG might appear to be, say, an M4A1 with silencer it might actually have a full M16 length inner barrel that runs the full length of the barrel and silencer.
Silencers come in a broad variety of styles, this one with S.T. Simth markings and arbitrary serials. If your weapon does not have Counter-Clockwise / Positive threading then you can always look into getting an adapter.