From: windows.microsoft.com/en-us/windows/libraries-faq#1TC=windows-8
Libraries gather folders that are stored in different locations so you can browse them in one place. Here are answers to some common questions about libraries in Windows 8.1 and Windows RT 8.1.
Where did my libraries go?
They're still there, and they work the way they did before, they're just hidden from the left pane in File Explorer by default. You can still get to them under Favorites by opening Desktop. If you customized your libraries and want to see them in the left pane, tap or click View, tap or click Navigation pane, and then choose Show libraries.