Per the description on iTunes - "VowelViz is an innovative speech mapping tool that displays vowel productions in real-time. With applications in speech therapy, accent reduction/second language learning, and home practice, VowelViz is a versatile and easy to use tool for practicing vowels."
When you open the app you will see the Vowel Quadrilateral (displayed below). As you all know, this chart represent tongue placement in the mouth during vowel productions. As your student produces each vowel sound, VowelViz captures their productions through the iPad’s microphone, then displays a bouncy orange ball to show them where their vowel productions are falling. Your students goal is to have the ball hover within the area of the targeted vowel sound.
Complete Speech offers a variety of tutorials, and how to videos on their website, check those out here. If you're interested in purchasing this app, you can find it here in the iTunes app store.