What does the Twitter number mean?

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Its a list of "tweets" with the same hashtags, that way, they can be tracked and used for statistics about a trendy topic.

"The hash mark (#) before a word in a post allows you to tag that post for that word. However, in order to get tracked via a hash tag, you need to opt-in and follow http://www.Hashtags.org.
If you then visit Hashtags.org, you can click on any tag and it will show you all of the posts that have been tagged with that keyword."