Library allows to store pieces of code so that you can easily reuse them later.
Visually Library is a dockable panel displaying set of folders and non folder items - snippets. Library allows you to save unlimited number of code snippets and categorize them in an unlimited folder structure.
Rapid CSS already comes with a few handy code fragments stored in Library. You might want to check those ones.
To show or hide Library pane, on the View menu, click Library.
The following topics provide more information: