The Raspberry Pi is a so called Single Board Computer. It is a complete computer at the size of a deck of cards. It has USB, HDMI, Ethernet, WiFi, Bluetooth and you can hook up a camera and all kinds of sensors and actuators (like LED’s and motors). There are multiple versions available, Pi Zero and Pi 4 probably being the most popular options.
The Raspberry Pi is used in education a lot so that makes accessible tutorials easy to find. And you can find a huge amount of projects, ranging from Bird Feeder cameras and do citizen science with Bird identifiers. Monitoring your energy consumption, building your own fancy LED light set, create media players (1, 2, 3), and, of course, automated cat toys. And many, many more.
The nice thing from the Raspberry Pi is that it is a complete computer, including a desktop like Windows and MacOS. This familiar way of working makes it easier to get to learn the new things: connecting sensors and actuators (motors, LED’s etc.).
The downside, compared to the Arduino, is that it is less dedicated. Meaning: because it is a complete computer, it has to make time for reading the input (mouse and keyboard) of the user and other things. This makes that it is somewhat less precise than the Arduino. The Arduino just has _one_ task and one task only (the one that you programmed on it). Having just this one task, it can be more precise, timing wise, to execute it. To be honest, for many projects that is not crucial, not many people for instance care if the LED strip turns on a few milliseconds later once you enter the room.