Data in Bluetooth Low Energy is organized around units called 'GATT Services' and 'GATT Characteristics'. To expose data to another device, you must instantiate at least one service on your device.
Adafruit's Bluefruit LE Pro modules support some 'standard' services, described below (more may be added in the future).
The UART Service is the standard means of sending and receiving data between connected devices, and simulates a familiar two-line UART interface (one line to transmit data, another to receive it).
The service is described in detail on the dedicated UART Service page.