Normally, you’d use the control panel on the top of the camera to check shooting information and change the settings – it uses less power and it’s often in a more convenient position. But you can press this button to display the shooting settings on the rear LCD instead, and this can be useful in low light or if you’re operating the camera at a distance where the top panel is hard to read.
The screen info remains visible until you press the shutter release. After that, the camera reverts to its normal display, and if you want to see info on the rear screen for the next shot, you have to press the Info button again.