Parity is a way of checking for errors. It makes sure that the data sent out by the CPU over the FSB are received correctly by the North Bridge, and vice versa.
Intel CPUs have had bus parity for several years now. It's a terrific feature, and complements all of the error checking done over other buses such as USB, SATA, networks, etc. IMHO the biggest glaring hole in the whole scheme is that in most desktop systems there is absolutely no error checking done for data stored in memory.