I believe it is the plastic, often quite big and slightly curved guard that sits between the rear cassette and the spokes of the rear wheel.
They tend to be found more often on road bikes and their purpose is to stop the chain unmounting and tearing your spokes apart. However, with a properly maintained chain and correct gear indexing they really aren't necessary.
Why a pie plate? No idea, but like many I find them ugly and 'pie' is presumably derogatory. Unless of course there is actually such a thing as a pie plate, on which one places a pie before devouring it?