A low gear combination uses a small gear on the pedals and a large gear on the rear wheel. With this kind of combination one revolution of the pedals moves the bicycle a relatively short distance.
A high gear combination uses a large gear on the pedals and a small gear on the rear wheel. With this kind of combination one revolution of the pedals moves the bicycle a relatively long distance.
If you pedal at a constant cadence (measured in RPM) then with a low gear combination the bicycle moves slowly and with a high gear combination the bicycle moves more quickly.
High gear combinations are good for speed and for downhill bicycling or bicycling with the wind at your back. Low gear combinations are good when you are going slowly, when you are accelerating, climbing uphill, or fighting a headwind.