A camera cheese plate is a customised utility bracket used for rigging a camera.
Camera cheese plates offer the strength and rigidity needed to set up a secure rig and allow you to add accessories (like battery plates or monitor brackets), and mount them onto the camera body for stabilisation. Cheeseplates - or cheese plates - are usually black, which helps to reduce any accidental off-angle or side reflections moving towards your lens (despite the fact it's normally at the back of the camera!).