You're told 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. Needs headphones or a wired input if you want to avoid the app hearing itself.
Pitch detection hears a pitch, not a string — playing the right note on the wrong string will also pass, and any octave of it counts. The string named in the prompt is what to aim for, not something the app can police.