it muliplies the speed of your cpu to get the speed it is now like a x11 multiplier x218 = 2400mhz +2.4gig. somthing like that anyway i think but i guess you already know that.
The FSB is the external clock, which also means the connection between the cpu and the northbridge. The internal clock, which goes through your cpu is multiplied because its within your cpu and it can run at faster speeds.