@eniko it's a bit difficult for me to describe since i've only heard about it myself recently, but it's a set of building blocks for building "anything" on any sort of system, be that applications or games, on embedded hardware or on top of existing operating systems
here's the site if you want to spelunk around that https://arcan-fe.com/, i really encourage you to watch the presentation when you can though, it does an excellent job of describing things

@eniko have you heard of arcan
https://www.youtube.com/watch?v=07nqZIFRDJg