Hello Gene, and welcome to Seven Forums.
Libraries let you access and arrange items from one or more folders quickly without moving them from their original location. Libraries themselves don't store your files or folders. Instead, they point to different locations on a hard disk or external drive attached to your computer. This gives you easy access to a category of files no matter where they are stored.
A library has more than one included folders
that display in them by default.
For example, in your "Pictures" library, it has the the C:\Users\(user-name)\My Pictures and C:\Users\Public\Public Pictures folders included by default.
This is why when you open your C:\Users\(user-name)\My Pictures
folder, that's all you see and not anything that was in the C:\Users\Public\Public Pictures
folder. Only in the library would you see the files from both locations at once.
Hope this helps,