What is Jellyfin?
Per the Jellyfin GitHub:
Jellyfin is a Free Software Media System that puts you in control of managing and streaming your media. It is an alternative to the proprietary Emby and Plex, to provide media from a dedicated server to end-user devices via multiple apps. Jellyfin is descended from Emby's 3.5.2 release and ported to the .NET Core framework to enable full cross-platform support. There are no strings attached, no premium licenses or features, and no hidden agendas: just a team who want to build something better and work together to achieve it. We welcome anyone who is interested in joining us in our quest!
What does this mean?
If you have a lot of digital media that you own and have the source file for, you can use Jellyfin as a catalog to keep track of shows watched, movies played, music listened to. Jellyfin gives your files a pretty face and a pretty way to organize your files. As well, Jellyfin allows you to watch the content that you own the source fil, to other devices in and outside of your home network. For example you can start the movie at home and pause it before bed. The next day you are at work and on lunch you want to continue the movie, you login to your selfhosted Jellyfin instant and boom, you continue on right were you left of last night.