I haven’t seen an app that does it really well like some libraries or ontologies do but I’m certainly not well versed with all of them. Back in the day I used Evernote which was at least a start, as you could create arbitrary hierarchies (nest tags within tags).

So ideally you would want to be able to nest tags like this:

news.politics.europe.denmark

of course another person might prefer the hierarchy

politics.elections.news.denmark

There’s no strict right or wrong here but often over time some consensus forms. Bonus points if there are equivalency classes, ie “recipe”, “recipes”, “cooking recipe”, and even the Spanish versions “receta” and “recetas” all refer to the same thing.

By meta tags I mean the ability to describe and classify certain tag groups. For instance “politics”, “cats” and “Hollywood” are content tags while the tags “English”, “Danish” and “French” are language tags. “PDF”, “MP3” and “HTML” are file format tags but “video”, “music” and “text” are content form tags while “2023”, “2004-04-03” are time-line tags

Meta categories allow you for instance to search for pages that are about the English language, but not necessarily in English and surely not written by people who happen to have the last name ‘English’. Now some systems encode this information inside the string of the tag itself like so: “language = English” or “topic = cats”, but I think the most elegant solution is really to let a tag have categories or tags on its own which describe what it’s used for (thus meta tags).