Cadence is the speed of your pedalling; a high cadence uses a high number of revolutions per minute. Similarly, a low cadence uses high gears that are spun slowly.
Generally mountain bikers use a lower cadence than road bikers. Although a high cadence is more efficient, a slightly lower cadence offers more stability on rough, technical trails. With practice, trail riders can learn to keep there stability at a higher cadence, increasing efficiency and endurance drastically. Bicycle manufacturers know all about cadence, that's why mountain bike's come with lower gears; for the steep uphills.
Experienced riders pedal with a cadence between 80 and 100 RPM depending on the terrain. 95 RPM is the target cadence, as it is the most efficient.
If you are on a high-speed descent on a bumpy trail you may notice that it is hard to keep up your high 95 RPM cadence. It is much easier to get smooth, effective pedalling if you use a lower cadence, especially if you can't stay on the seat and have to pedal standing up. Just as a novice has a hard time being smooth at 100 RPM, the disruption of a brain-joggling trail descent can make an experienced rider retreat to slower pedalling for the sake of getting a smooth, controllable ride.
Cadence is very important for making it up those big uphills. Using a proper cadence will mean you will make it to the top of that hill instead of burning out half way. See Mountain Bike Climbing for more about climbing.
It is important to remember to shift before you get to an obstacle or hill. Sometimes, high quality shifters will let you get away with poor shifting technique, but if you are on anything less than XT make sure you are ready ahead of time.