The Raspberry Pi is a series of very affordable credit card-sized single-board computers developed in the United Kingdom.
As it is described in this awesome GitHub repo, the sky is the limit when it comes to the projects and the use cases of it, it all basically boils down to your creativity and needs.
As of June 2020, the most advanced version is Raspberry Pi 4 Model B, offering up to 8GB of RAM, USB-3 ports, Dual Micro HDMI ports (supporting 2k and 4k displays) and other superior features.
Do you already own some? What is/would be your use of Raspberry Pi?