Capacitive touch sensor switch module - This type of device uses your body as part of the circuit. Once the touch sensor pad is touched by your hand or any part of your body, the capacitance of the circuit is changed and the sensor will detect this. The detected change in the capacitance results in the output changing the states of the device.
This module consists of three pins, VCC for the voltage source, GND for the ground and SIG for the signal. The sensor pad is located in the center of the module and it also has a built in LED.
The Sensor in this video from another Manufacturer but it's the same