it will keep your pci/agp speed at 33/66mhz. this is very important when fsb overclocking because many devices do not run out of their pci/agp spec. its a good thing to enable this even when your not fsb overclocking, as in the case of amd overclocking.
The PCI/AGP lock holds the bus speed at 33/66Mhz while you fiddle with the FSB. This helps when OCing so any cards plugged in don't hold you back by not being able to run out of spec.