Your raspberry pi has a set of bi-directional input/output pins that allow you to interact with the physical world. There are two ways to address the numbers: Broadcom (BCM) pin numbering and board numbering. Board numbering goes from 1-40 corresponding to the pins on the device. Any pin marked GPIO can be used for input and output. Some pins are reserved as power outputs, electrical grounds, or for other purposes.
Serial Peripheral Interface (SPI)