I'm not sure what we're supposed to put under "Enter Tag" for a map in the file share. A short description of the map? The available game types? Size (S, M, L?) or number of players? Key words? If so, what? What is this for, exactly?
Tags are keywords users or yourself places on an item of custom content to make it easier to find. Example: Ive made a funny screenshot. I tag it funny, so when someone searches "Funny" looking for screenshots, mine comes up. The more tags you put on your item, the easier for someone else to find it, which usually ends in more downloads. So tag your items.
Note also that on Bungie.net, different tags are separated with semi-colons. In-game, you must enter them one at a time. (Don't try separating tags with spaces in-game; you'll just get "small koth hill" all as one tag.)
Also you might get author rams [insert inanimate object here] tags. It gets really annoying after awhile.