HDR stands for "high dynamic range," and enabling it will give you brighter, sharper, and more well-lit images. Tap "HDR On" at the top of the screen in camera view. There are a few dos and don'ts, but for the most part, HDR will give you the best picture possible.
It essentially takes a series of images, each shot with a different exposure from darkest to lightest. HDR combines the best parts of the three overexposed, underexposed, and balanced shots to create a dramatic image with beautiful shadowing and highlights.