The hash tag (#), is used on Twitter for users to categorize their tweets. By using hash tags that place your tweet into an appropriate category, it would be easier to find by users who know how to search for hash tags.
Ok, let's break this down a bit... First of all, hash tags can be made up by users -- it is really an "unofficial" categorization method developed by the community. So for example, if your post is about pokemon, you can include the has tag "#pokemon". Now, if you wanted to know what was going on at Nats, you can search for "#pokenats10" at search.twitter.com and see all the related posts, assuming users who tweeted about it used the tag.