AUTO without Flash
Explanation: This mode is identical to AUTO except that the D5100 will never use the flash, not even in very low light.
Recommendation: If you use the AUTO mode you should also experiment on this mode to suppress the flash.
Using available light instead of the harsh flash light produces a lot more atmospheric pictures but has the risk of blurred pictures in low light.
To learn when to use the flash and when not is one of the first steps on the way to taking better pictures.