FaceRange detects faces from your phone camera and use that information to calculate estimations of distances from you to that person. If the person is too far away and can't be "seen" by the face detector, you can use the zoom slider at the bottom of the screen to zoom in on him/her to get a better result.
Please be noticed that although the developer try very hard to get the best out of face detection information, it is still far from accurate. The results are only rough estimations and should not be taken as serious measurements.
Due to the limitation of hardware implementations, not every phone support face detection in ICS, therefore if you see a message saying face detection not supported when you start the app, that means your phone does not support this app.