It’s a media server that lets you access movies/tv shows/music from other devices via apps or a web interface. In this post’s case it’s talking about the Jellyfin app on Roku that lets you watch/listen to stuff through Roku devices.

In my experience it’s been pretty nice and easy to use, and the Roku app’s decent enough for what I’m after, which is simply watching stuff from my PC on other tvs in other rooms.

Edit:
Btw here’s a link to the post OP screencapped: https://social.linux.pizza/@tgpo/111551351394147888

https://jellyfin.org/

Jellyfin is a FOSS alternative to Plex. In simple terms, is a media streaming software. Download and host your media on one computer, stream it to other devices

Who are you? Who am I? Wait, is this a dream?

Seriously, though. Jellyfin is the bees knees