Once the app is downloaded, it can be activated within the 'accessibility' menu on your device, allowing you to turn on a series of features made for anyone who has a hard time seeing what's happening on the screen of their Android.
One feature, for example, enables you to listen to a description of absolutely everything that you touch on the screen. Just pass your finger over any element, and you'll hear the description.
Google Talkback is a tremendously useful tool for anyone with vision problems. Once you install it, you should have a much easier time navigating your Android device.