Originially file systems did not have any hierarchy. All Files were just kept in a plain list. Some people found that limiting and created trees of folders. Other people considered files to be the wrong abstraction and used databases instead.
Tagga is a bit of a middle ground. Instead of sorting files into a tree of folders, they are tagged with a number of attributes. For retrieval they can be filtered, much like a SELECT ... WHERE statement does.
This idea is backwards-compatible, as the path can just be an tag.
Right now its just a prototype to explore the ideas viability. It uses a database for storing the tags, and a folder to store the files.
In the future it would be nice to have an in-kernel implementation. It would then need to store the tags in the file system (hard) and the files would just become inodes (easy)