Press "Play Melody" to start. You will hear a randomly generated melody from the C major scale, with one note out of tune. Select which note you think is out of tune and by how much, and you will get some number of points based on how close your guess is.
This game is hard, but that's okay. If you just struggle with it for a while then pretty soon you'll be good at it. The purpose of this tool is to improve the precision of your relative pitch so that you can correct your intonation better, especially when singing. Note that this is different from learning perfect pitch, since you only need to be able to recognize the notes in context.