This is a piezoelectric shock sensor disc, 20mm diameter, with connection wires and a 1MΩ load resistor sealed with resin. The wires have male pin ends so you can plug this into a solderless breadboard for quick prototype work, e.g. in an Arduino project.
With this sensor, you can detect footsteps if it's placed on the floor, or create a MIDI drum, a door knock detector, etc.
This is an inexpensive device. No user manual or datasheet is provided. When hooked up to an oscilloscope, the amplitude of the generated voltage has shown to be about 0.05V with light knocks near it, and up to 3-5V when hit directly on the sensor. The 1MΩ resistor discharges it for about 25-40 milliseconds.
Download Example Arduino Code — one for use with a servo; another to set up and use a knock-pattern password.