What exactly is this? I couldn't find the answer after browsing through the wiki and FAQ. I'm assuming it has something to do with the tags that a user creates?
(I realized that I failed to properly make artist pages for a good number of my uploads, so I was planning to fix that now. If the "Uploaded Tags" feature lets me see all the new tags that I created, it would tremendously help make the process less painful.)
It's basically a list of tags meant to help you tag posts more efficiently. You can let Danbooru calculate them automatically from the most commonly used tags in your tag edit history or add the ones you want manually in the Uploaded Tags box in the settings page, and afterwards they will appear when you edit posts.
Edit: Algasir's right, apparently you can't let Danbooru calculate them automatically anymore. Manual input still works though.
Yeah, I did my list manually - my most uploaded tags don't really include duplicate or jpeg_artifacts, I swear. =P So "uploaded tags" isn't really the right term, it's more like "quick tags" or "bookmarked tags" or something.
Manual here as well. I use it for things I find myself adding frequently, or sometimes for things I want to remind myself to watch for (pigeon_toed, blush_stickers). Very handy.