A word, phrase or topic that is tagged at a greater rate than other tags is said to be a trending topic (TT for short).
Trending topics become popular either through a concerted effort by users or because of an event that prompts people to talk about one specific topic.
These topics help Twitter and their users to understand what is happening in the world.
Trending topics are sometimes the result of concerted efforts by fans of certain celebrities or cultural phenomena. The trend algorithm has been altered in the past to prevent manipulation of this type.