They are both media servers. Plex is closed source and requires you to make an account with them to use whereas jellyfin is open source and requires no account.
Basically when you set them up you just pick a directory for movies and tv shows and any video files in that directory can be streamed thru the app or web ui
I prefer jellyfin because of the differences i mentioned earlier but i use both (you can point them at the same directories). I haven’t really had any issues with jellyfin performance but run plex as a backup.
Plex has plex shares too where you can stream other peoples content from your server. Idk if jellyfin has that feature
Since i’m a console player mainly i find myself trying to use a controller when playing pc. However, i recently started playing half life 1 which didnt have a controller option and its gotten me more used to the idea of using keyboard and mouse