What does a gear box do?

The different gears in the gear box allows you to trade power for speed, or the other way around.

The engine only has a certain amount of turning power(torque) and a certain amount of turning speed(RPM, revolutions-per-minute), so you need the gears to make a useful vehicle out of it.

To get the car going from from standstill, you need a lot of turning power, so you use a low gear which makes the car strong but slow.

Now, if you stayed in the low gear you wouldn't be able to go very fast, because the engine would soon be turning as fast as it could.

So you go to a higher gear, trade a bit of power for speed.

It's a bit like walking uphill really. Taking small steps makes you strong, but it takes forever. Taking big strides lets you pick up speed, but if it's steep it's really tiring.