You get a note and a string. Find it yourself and play it. The app listens through your microphone or audio interface, and moves on when it hears the note. Use headphones or a wired input, or the app hears itself.
The app hears a pitch, not a string. The right note on the wrong string passes too, and so does any octave of it. Aim for the string in the prompt, but the app cannot check that part.