Usually that refers to the width of the data bus used to transfer information between the graphics processing circuits and the graphics memory. For two video cards that have the same architecture, one with a 192 bit interface and another with a 256 bit interface and all other things equal, then the one with the 256 bit interface can transfer 33% more inormation in the same amount of time. However, that does not mean it is 33% faster because the graphics processor still operates on the data at the same rate. It often does improve performance but not as much as you might think.