Auto Exposure Mode gives you the option to have the camera adjust its exposure based on changing lighting, or to keep it fixed to its original exposure level. You should consider keeping this enabled if you will be in a changing environment, such as an outdoor event at dusk.
Go to Settings, then tap Advanced Settings.
About halfway down the Advanced Camera Settings section is Auto Exposure Mode with two options.
Auto-Adjust is selected by default.
Fixed will not adjust your camera's exposure. This is ideal for a settings that has set up lighting for a specific effect, such as a concert.
Tap whichever option you prefer; it will turn blue when it's selected.
Closing out the menu will save your settings.