PICO-8 is like a small editor with everything you need to make a game, but has a lot of limitations, like 128x128 pixel, 16 colors, limits on how many lines of code you can write, and limits to memory. You also only have arrow keys and two buttons.
They say it's a fantasy console, because developing for pico8 is like developing for an old console like nes/snes.
I find it cool especially because of the editor that gives you all the tools, its simplicity driven by its limitations, and the fact that it export for web and is also playable from mobile (the web version).
You can find more info in PICO-8 home page, here.