"Tags" is a word for the subject matter of a thread. It is an internet system that originated, I am guessing, so that people can find threads if they look a subject up on google.com. Thus, you could have tagged this thread of yours as "forum usage", or "how to use a2k". People have by now tagged it for you.
I might tag a cooking discussion thread of mine as "baking","bread", "yeast bread", "no knead bread". That way, someone looking in google for different kinds of baking threads, or for very specific kinds of bread baking threads, will find that a2k addresses those subjects.
The reason the box was blank was that you didn't add tags to the box when you clicked on the little green words "tag this topic". You can also tag your thread when you fill in the question or discussion form - first, the thread title, next, the thread's opening post, and, finally, the tags you'd like to add.