Thread: Tag.

You can search for threads on here and tags make threads that correspond to your search more likely to show up. Therefore when you add a tag, it should correspond to the subject of the thread. The search panel is right beside the "log out" button.

For example, if I searched up "tag" in the search panel, I might see this thread appear because of the "tag" tag that you put on this thread. However, in this particular instance, that is most unlikely because of it is such an ambiguous term that is probably adjoined to a wide spectrum of topics. When you add tags, you want to narrow it down to the exact subject to yield the most definitive search results.

Think of it as a way to "categorize your topic".