A library gathers content from various folders. You can include folders from different locations in the same library, and then view and arrange the files in those folders as one collection. For example, if you keep some of your pictures on an external hard drive, you can include a folder from that hard drive in your Pictures library, and then access that folder's files in your Pictures library whenever the hard drive is connected to your computer.
The navigation pane, showing the Pictures library with three included folders
To learn more about what types of folders you can include in a library, see Libraries: frequently asked questions