Mock suppressors/silencers/barrel extensions in airsoft are a cosmetic upgrade to your airsoft gun to enhance its looks. They do not change the way your gun performs or behaves and they do not "silence" technically speaking. All barrel extensions legally sold in the US are required to be solid and are not to have any noise suppression capabilities.
That being said, these things still look very cool and are a nice way to dress up your airsoft gun. Some people will hide a longer inner barrel inside them so you have the "suppressor" look but the performance gains of having a longer barrel.