What does the #insert word here mean in twitter?

[Light]

[sabrina]

It's a hashtag! If you hashtag something then if someone looks up what you hashtagged, your post will come up! For example if I wrote #hey then if someone looked up #hey, then your post would be one of the posts that came up.

[Erik Van Thienen]

It's called a "hashtag", they are put before important words, and provide a means of grouping such messages, since one can search for the hashtag and get the set of messages that contain it.