my car is automatic, can you answer?

The lower gear ranges are useful for acceleration like the previous poster stated but are generally used for decelerating down hills, etc. Whenever you travel through an area that has a lot of mountains or steep hills, you will want to use the lower gears to slow your car instead of the brakes. Unfortunately, not a lot of people know that and use their brakes for prolonged periods of time. After a while, your brakes will get too hot and will "fade" and could possibly stop working altogether and that could be disastrous! By using your lower gears, the engine uses its own compression to slow the car, essentially acting as an engine brake.

They represent different gears. These lower gears will give you considerably more torque. D3 is generally used for slight up hill driving but D4 will do the trick. D2 and D1 are used for steep uphill driving and when going downhill these lower gears slow your car down when you have your foot off the gas and brake. These gears take a lot of torque to move them 1 revolution when compared to the higher gears thus making the gear great for uphill driving. You will notice the difference when you go uphill in D4 and then in D2. The D2 will feel like it has more power.