At its most basic level, communication between two pieces of hardware requires the transmission of data back and forth between them. In computer systems, this data is represented as voltage levels sent over a wire that connects the devices together. The patterns and levels of voltage back and forth form a communication protocol that the devices use to transmit data between each other.
GPIO is the most basic interfacing option offered by microcontrollers and microprocessors. Some pins of the BBB's processor are allocated as GPIOs that act as an input (monitoring voltage on the wire to receive data) or an output (placing a particular voltage on the wire to send data). The BBB has dozens of available GPIO pins, which makes GPIO a flexible and simple way for Android apps to interact with the outside world without requiring fancy device drivers or extra interfacing hardware.
