Windows libraries are handy, as they let you group together scattered bunches of similar kinds of files in a single logical, themed place for easy access. For example, if you store your music both locally on your PC and on a network drive, you can access both locations from a single library.
By default, the File Explorer in Windows 8.1 doesn't display your libraries, but you can easily fix that. Open File Explorer from the desktop, select the 'View' menu, then select 'Navigation Pane'. From the drop-down list, check the option to 'Show libraries'.