Depending which SUV
The 2 gear on your selector is to stop your vehicle tranny from changing into higher gear then 2nd.
L is the lowest gear you can select if you need on the selector and they are used for towing loads from standstill or not wanting to go into a higher gear.
Overdrive is the highest gear that your auto tranny has which allows it help in fuel consumption and gain higher speeds.
Do a search you will find a lot more then what I just said.