XBee devices communicate with each other over the air, sending and receiving wireless messages. The devices only transfer those wireless messages; they cannot manage the received or sent data. However, they can communicate with intelligent devices via the serial interface.
XBee devices transmit data coming from the serial input over the air, and they send anything received wirelessly to the serial output. Whether for communication purposes or simply for configuring the device, a combination of both processes makes XBee communication possible. In this way, intelligent devices such as microcontrollers or PCs can control what the XBee device sends and manage incoming wireless messages.