If a folder has a "+" sign on it, then it contains subfolders and / or files.  If a folder has no markings on it at all, then it is empty.
Click on a folder with a "+" on it to expand the folder and view its contents.  Upon being opened, a folder's "+" indicator will change to a "-".  Click the folder again to close it. 

Once a folder has been opened, any subfolders will be displayed in the main File Library structure, and you can navigate through them in the same way as you perused the top level of folders. 
If a folder contains documents, you may click on the document's title to view information about the document and / or view or download it.