The idea, says Bidelman, is to make the HTML5 API "more approachable for developers that have done file I/O in other languages" as well as making common operations like renaming, moving and duplicating files easier. To that end, the library uses familiar UNIX-type commands (cd, cp, mkdir, mv, rm and others) and accepts entries in several formats.
Bidelman has also included sample code, tests and a sample app for testing out filer.js.
Currently, the HTML5 Filesystem API is only implemented in Chrome, so you won't be able to make use of filer.js in other browsers. If you're not familiar with the HTML5 Filesystem API yet, there's Bidelman's book as well as a tutorial on exploring the filesystem APIs on HTML5 Rocks.