With GPIO subsystem, you can manage the general purpose input and outputs available on Carberry main connector.
GPIs can be used to read pushbuttons, microswitch, sensors, and so on, by Raspberry Pi Linux software.
GPOs can be used to drive external leds, relays, actuators, and so on, by Raspberry Pi Linux software.
Please, refers to Connectors Specifications for more informations about electrical constrains.