On the Start screen, swipe left, and tap Settings > Brightness.
Automatically adjust is set by default, then the light sensor observes lighting conditions and adjust the illumination of the screen and bottom keys to the environment. When you switch off the automatic brightness control, you can choose between three levels low, medium and high.
When using high brightness settings or when displaying white or vibrant colors the screen will use more energy. Obviously this will also influence battery life. Also, note that the battery saver mode is to reduce further the screen brightness for saving battery.