iOS has a lot of great features, some of which are focused on preserving as much battery life as possible. One of them is the Auto-Lock feature, which defaults to dimming the screen of your iOS device after a really short period of time.
This obviously helps preserve battery life, as the less the screen is on at full brightness, the longer you’ll be able to use it on a single charge. If you’d like to control this, though, and even turn it off fully, it’s a simple trip into the Settings App.