1. VCC Used to power the module. Connect it to a 3.3V pin of the microcontroller.
2. GND Connect this pin to the common ground of the circuit.
3. TXD Connect this pin with the RXD pin of the Microcontroller. This pin transmits Serial data (wireless signals received by the Bluetooth module are converted by module and transmitted out serially on this pin)
4. RXD Connect this pin to the TXD pin of the Microcontroller. The HC-06 Bluetooth module receives the data from this pin and then transmits it wirelessly.