PRNDL—Park, Reverse, Neutral, Drive and Low. Some gearshifts will have L, 2, 1, or 2, 1, etc after D. Most automatic cars have two or three options after Drive. Do you know what these positions are for? Have you used them? Do you know when to use them? The answer is in your owner's manual but here is a (very) brief explanation.
There's a close relationship between your car's power and speed. Generally, more power means less speed and vice versa. Let's say your car's gearshift has PRND21. When you shift into 2, you'll get more power but remember that you'll have less speed. It's useful when you're towing, driving on hilly roads or driving in deep snow/mud. Some transmissions will stay locked in second gear while others will automatically shift out of that gear when you reach a certain speed to prevent damage. Either way, you're controlling the transmission more so than when you simply put it into Drive. When you manually select 1, you're getting even more power but the vehicle will be moving slower. Also keep in mind that fuel economy is much less in these lower modes compared to 'D'.
Don't drive the vehicle in these selected positions beyond their speed range. For example, our 2004 Toyota Corolla has a speed range of up to 118km/hr in 2 and up to 65km/hr in L. Going beyond the speed range will increase the chance of damaging the vehicle, and/or overheating the transmission fluid. If you find yourself going beyond those ranges, it's time to shift into a higher gear (or 'D' if applicable).
These manual options serve a good purpose when used for the correct situation (driving on steep hills, in extreme weather conditions, etc). Before you start selecting these options, it's very important that you read your owner's manual. It will give you specific information on when and how to use them as well as what the speed ranges are.
Image via Honda Canada (2013 Civic)