It's simple, really. The app uses the camera on the back of the phone to turn the background of the screen and keyboard into an image of what's in front of you. So you can hold the phone up in front of you and look at the keyboard while still seeing what's in front of you.
The only downside? This will prevent you from ever running into a pretty girl on the street who is also texting while walking, giving you an excuse to talk that will perhaps lead to a relationship that could end in a lifelong happy marriage. But really, it'll probably just keep you from walking crotch-first into a parking meter, so install away.