Cadence refers to the number of crank rotations per minute. A lot of computers will give you a rpm readout, but it's not necessarily accurate, these are usually reading wheel rotations, not crank. Anywho...the idea is that most racing cyclists have a cadence between 80 and 120 rpm. Most recreational riders average between 60-80.

I used to have very low cadence (around 70) until I started racing. Now, my cadence is usually upward of 80-90. It has a lot to do with fitness, but also knowing how to use the gearing to your best advantage to conserve energy and keep you moving.