rpi.gpio refers to the package name of a Python module for accessing the Raspberry Pi's GPIO pins. The module allows GPIO pins to be initialised, turned on, and turned off. It uses a functional programming style.
For more information see the rpio.gpio homepage.
If you are not aware, there are a number of different pieces of software that serve a similar fundamental purpose (controlling the Raspberry Pi GPIO pins) and are independent of one another. Not surprisingly, they also all use spins on combining "rpi" and "gpio" in their name. Each has a separate tag and questions about their use should use that specific tag. This is documented here to prevent confusion.
- The page you are on now is for the
RPi.GPIOmodule for python: Tag rpi.gpio
pigpiolibrary, written in C but with ports to python and other languages: Tag pigpio
This isn't intented as an exhaustive list of such software, just the three most prone to confusion here due to the similar names perhaps made more similiar by restrictions on tag labels (all lower case, etc).