It's the speed at which the processor and other components can pass data across the motherboard.
It has an impact on the "speed" of your PC. If you fit a faster processor, the PC should speed up.
BUT, once the processor has to wait while data goes through that bus, it limits the speed of your PC and the processor's extra capacity os wasted.
INTEL and AMD processors have an internal bus-side controller that looks after this. In the AMD devices it sits in a different location to the INTEL. In many situations, it can pump data around more quickly, which is why AMD say that their devices are equivalent to a higher-speed INTEL device.
However, this is only true for certain types of data movement.