It is a very popular Single Board Computer, with a lot of community support that allows people to build and program a variety of things for a low price. Think of it like lego, but for things which can be useful as well as fun.

Want to run a weather station? Pi and a couple of off the shelf sensors, done.

Want to control your lights or appliances from your phone without getting out of bed? Pi and a couple of off the shelf relays, done.

Want to build a retro gaming console? Pi, a couple of off the shelf controllers and some pre-made emulators, done.

Tiny computer about the size of an Altoids tin targeted at hobbyists and students.