FSB Stands for Front Side Bus. This term is used with regards to the Frequency Capability of Internal Data Transfer built in the Motherboards and other components. The FSB of the Mother board and CPU Frequency should be compatible for them to work together. Mother Board with FSB of 1333 will be able to Accept a CPU with 1333 or Lesser frequency (e.g., 1066 or 800.

In General terms, Higher the Frequency - Faster the Speed, More Expensive, More Productive.