PICO-8 is a fantasy console, a small computer program that emulates a video game console that never existed. It focuses on allowing users to both create and play video games as part of a single experience, with the Esc key switching between the two modes.
It intentionally calls back to retro aesthetics with a 128x128 pixel screen, only 16 different colours, and a fairly limited maximum game size. However, it simplifies actually creating games, opting to use a stripped-down form of Lua for game code, as well as integrated sprite, map, sfx, and music editors. Games can be exported to all popular operating systems and very easily embedding into webpages. The end result is nice retro experiences that are simple to create and share with others.
In PICO-8, a cartridge is a collection of code, graphics, and music that can be played. A cart can be a game, a music disk, or any other sort of interactive or non-interactive experience. The 'virtual BBS' on the PICO-8 website lets users upload their carts, and the SPLORE interface in PICO-8 lets people easily browse and play anything that's uploaded to the BBS.
Because of their size, PICO-8 carts can be exported as small PNG image files with the cart information embedded. This let people use standard image hosting websites to share their carts with other users, rather than relying on the BBS or more obscure hosting services.
One of the most interesting features about the PICO-8 platform are the limitations. There's no Lua standard library, your entire sprite sheet can only be 128x128 pixels in size, and you have a limited amount of code (8192 tokens or 65535 characters, whichever is smaller).
It's common for people to spend months or even years on personal projects, especially in the informal video game development space. While PICO-8's limitations and boundaries do change what types of games can be created, they also force people into a comfy creative space where the scope stays small and where shonky, unpolished work is fine (similar to the fanzine community). The PICO-8 community is also closely linked to the demoscene, with many sceners creating and submitting carts as entries to demoparty compos.
PICO-8 has also inspired a number of similar fantasy consoles such as BASIC8, LIKO-12, Pixel Vision 8, and TIC-80.