A "tag" is bit of information, or metadata, that is associated with a data file. Specifically, tags are keywords or terms that you feel have some relevance to a particular soundfile. Ardour can store these tags in a seachable database so that you can quickly search for sounds based on the tags that you have assigned to them. For example you can assign the term "120bpm" to a sound, and then when you search for this tag, the file will appear in the search list. Tags are independent of the filename or anything else about the file. Tags, and the file paths that they are associated with, are stored in a file called "sfdb" in your Ardour user folder.