What's #term/word/phrase command mean on twitter?

[Andrey]

You know when people put "#" in front of a word on their updates. like #swineflu, #celebrity, #blahblah, #yadayada...you understand? What does that command mean? I can't find it on their help page.

[ig4mer]

# = Subject or a conversation. So I can put #mets on all my messages to indicate I am talking about this subject. #Mets lost 5-4.
This is used to search for conversations. without the # tag you would only see tweets of people you follow but with the # tag you can search twitter for something like #Mets so see everyone would talk about this subject. This is especially effective when something is happening real time like the mumbai shootings with the #mumbai tag or the swine flu.