The Auto-Lock option on iPhones and iPads dims then turns off the display if you haven't tapped anything in a while. It's a battery saving feature.
But sometimes, say, when you're looking at a timer in Timer+ counting down from across the room, you want the display to stay on even if you're not tapping around on your iPhone or iPad.
You can prevent this auto-dimming of the display by setting Prevent Auto-Lock to ON. Note that this option will apply ONLY to the Timer+ app when it's running in the foreground. It will not override the system Auto-Lock setting.