FSB stands for Front Side Bus and for all intensive purposes it is the link between the cpu and the motherboard. When you overclock you increase the FSB speed (eg Pentium III 800E default = 8x multiplier multiplied by 100MHz FSB = 800MHz => 8x x 133 = 1064)
To overclock your FSB, you need to go into your BIOS/System setup (Usually your PC will say when it boots Press [Del] to enter setup), so do what it says. Then look for a "Manual CPU adjustment," "FSB Adjustment," or "CPU SoftMenu." Then adjust the FSB and voltage from there.
Increase the FSB until the system is unstable, then turn it back a notch... if it is only just unstable, increase the voltage some.
And there's Chris's crash course in overclocking - hope it helps.