The idea behind all these gears is to allow the rider to crank the pedals at a constant pace no matter what kind of slope the bike is on. You can understand this better by picturing a bike with just a single gear. Each time you rotate the pedals one turn, the rear wheel would rotate one turn as well (1:1 gear ratio).

If the rear wheel is 26 inches in diameter, then with 1:1 gearing, one full twist on the pedals would result in the wheel covering 81.6 inches of ground. If you are pedaling at a speed of 50 RPM, this means that the bike can cover over 340 feet of ground per minute. This is only 3.8 MPH, which is the equivalence of walking speed. This is ideal for climbing a steep hill, although bad for ground or going downhill.

To go faster you'll need a different ratio. To ride downhill at 25 MPH with a 50 RPM cadence at the pedals, you'll need a 5.6:1 gear ratio. A bike with a lot of gears will give you a large number of increments between a 1:1 gear ratio and a 6.5:1 gear ratio so that you can always pedal at 50 RPM, no matter how fast you are actually going.

On a normal 27 speed mountain bike, six of the gear ratios are so close to each other that you can't notice any difference between them.

With actual use, bike riders tend to choose a front sprocket suitable for the slope they are riding on and stick with it, although the front sprocket can be difficult to shift under heavy load. It's much easier to shit between the gears on the rear.

If you are cranking up a hill, it's best to choose the smallest sprocket on the front then shift between the nine gears available on the rear. The more speeds you have on the back sprocket, the bigger advantage you'll have.

All in all, gears are very important to mountain bikes as they dictate your overall speed. Without gears you wouldn't be able to build speed nor would you be able to pound pedals. The gears will move the pedals and help you build up speed.

There are all types of gears available in mountain bikes, all of which will help you build up a lot of momentum if you use them the right way.