On twitter, what does it mean in trending topics when there is #_________?

[L.B.]

On twitter in the "trending topics" section alot of times there is # then a phrase like #howdareyou or something.. what does this mean?

[Nyral Nerdysaur :0]

Its just a tag.
Like when you put @___ to show the tweet is to a specific person.
You put #___ to show that it's in a certain topic.

[Boboo]

keyword are called hashtags. Hashtags are tags that show that a tweet is about a specific topic.
Read the article below for an in-depth explanation.