What it says on the tin. This works for both logged in and logged out users, but logged out users will only be able to see the tag IDs for canonical tags with RSS feeds on them (so Fandom, Character, and Relationship tags), while logged in users can see all three plus Ratings, Warnings, Categories, and Additional Tags.

Not really as useful anymore since AO3 rolled out the new filters with Include/Exclude (which is awesome!), but this is still helpful if you want to do OR filtering instead of AND.