This app don't produce sound , it receives sound from real guitar by microphone and tell user if the guitar string is tune well by moving a small triangle mark to the corresponding string on the screen.
if triangle is pointing to the left of string you play, that means you should tighten it.
if triangle is pointing to the right of string you play, that means you should loosen it.
The tuner reacts faster and more accurate if the microphone is placed closer to the string.
Touch the other strings with your fingers to prevent them from producing sound and disturbing the measurement.