You will see a page to add libraries when you first create the server, but they can also be added or removed at any time from the settings. Libraries are virtual collections of media and can contain files from several different locations on the server.
The three most common types of content are movies, shows, and music. These will have the best support in client apps. You can also add other types of media such as books or photos. If you have several types of media in a single folder you can also label it as mixed, which will be a generic folder view that displays all files in the library.
You can add multiple paths that will all be shown under the same library. The path selection dialog will allow you to select folders visually, but if you can't find the exact location you can also just enter the path manually.
Real Time Monitoring¶
This will let Jellyfin automatically update libraries when files are added or modified. Unfortunately this feature is only supported on certain filesystems.