Air restrictor are small parts that are inside most Nerf blasters. They prevent most of the air from escaping the mechanism and worsen accuracy. Because of this, Nerfers will open up their guns and remove them. This method is called "Blaster Modification", or a "gun mod".
Extensive use of the blaster without the air restrictor can damage the internal mechanism because the air restrictor also protects the internals. To compensate for this modders place rubber padding to protect the blaster's internals.