well the idea is to increase the clock speed of the cpu. With an increased clock speed it does more work in the same time, so the performance is increased. And multiplier x fsb = clock speed. Like the stock clock speed of my cpu is 3.00ghz (multiplier = 15, fsb = 200). But I increased the fsb to 225 so now I'm running at 3.38ghz.
Pros: better performance
cons: more stress on the cpu&ram, higher temps (both of these can lead to the cpu braking, but it's rare and won't happen if you know what you are doing) increased electricity consumption