A Piezo Sounder is a Piezoelectric speaker and is used to convert electrical signals into sound just like a speaker. It can be used to play musical tunes or create sounds.
The Piezo Sounder is generally used to generate electronic sounds directly from a microcontroller without any amplification circuit. It can be used as a beeper to indicate the success after a specific action for example beep after a button press was recognized. You can even play whole tunes on it.