It’s like a general purpose expansion for the RaspberryPi. Let’s you hook up all kinds of things like GPS receivers, lights, displays.
Discussion
What can you do with a raspberry pi? I know it’s a computer.
They used to be really popular for maker stuff like displays, robots, controlling things but a lot of those uses have been replaced with other microcontrollers.
You can also use them as really low power servers. That’s what I was doing with mine. Pi-hole is a popular is a popular use which is a network wide ad blocker.