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 searchable 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.

To add tags to a given file, open the Session > Import dialog, select the file in the browser, and type new tags into tag area in the soundfile information box on the right.
Tags are stored when the input box loses focus, there is no need to explicitly save them.

You can search for specific tags in the Search Tags tab of the same dialog. Files which have been tagged with the relevant terms will appear in the results window. Selected files can be auditioned and marked with additional tags if required.