Gearing on a bike can be analagous to gearing on a car, with a few minor differences. This might make more sense to folks with manual transmission, but here goes:

Easier gears are for starting out or riding through difficult terrain (uphill, through mud, gravel, etc).

Higher gears are for moving at speed, but here's where the difference comes in-- In your car, once you get into high gear you stay there, unless you reduce speed. On your bike, higher gears are used to apply more power to your pedaling, enabling you to sprint, or for pedaling super-easy terrain (like going downhill). Your middle gears are the ones you'll most likely use for maintaining a consistent and comfortable cadence/speed on neutral terrain.

I hope that makes as much sense here as it does in my head