Piezo element is a sensor which is used to detect vibration, in this case, a knock on a door, table, or other solid surface.
A piezo is an electronic device (sensor) that generates a voltage when it's physically deformed by a vibration, sound wave, or mechanical strain. Similarly, when you put a voltage across a piezo, it vibrates and creates a tone. Piezos can be used both to play tones and to detect tones.
For more information on piezo-electric elements, please click here
For how to use peizo sensor with Arduino, please click here