An intake restrictor plate is a metal plate with a given-sized hole in it introduced into the air intake system of an automobile to limit performance in auto racing. As peak power in a vehicle comes from peak efficiency, which in turn comes from having a stoichiometric air/fuel ratio, this limits the amount of air which your engine can take in, which in turn limits the amount of fuel which can be burned, thus limiting the amount of power which can be developed during combustion.
Restrictor plates have always been a subject of controversy in racing, but they do tend to even the pack. A great deal of work is done by factory racing teams in order to help mitigate restriction, such as using CFD (computational flow dynamics) modeling and creating controlled vortices inside the intake in order to compress intake air as it passes through the restrictor, thus allowing more air into the engine.