This item includes two HC05 Bluetooth modules compatible with Arduino or other microcontrollers. These modules are pre-configured at the factory so that they automatically pair with each other on boot. One is in master mode, the other in slave mode. The UARTs on the modules are set to 38400 baud which is a good speed for Arduino (if you run much faster there can be issues.) The modules are marked with a unique ID so you can tell which ones pair with each other if you order more than one pair. The master is also marked "M" and the slave is marked "S" in case that matters in your application. Instructions and sample code are posted on the vorpalrobotics.com website.