If someone uses a hashtag (#) in a topic thread, the hashtagged word will change to a hyperlink, linking users to the search to find posts with that specific hashtag word. This is a way for users to group topics together using a unique word/phrase, most famously used by Twitter.
Examples:
This bitcoin wallet just made a cool update that makes transactions so much easier! #Wallets101
Now this is how you make a bitcoin wallet. #Wallets101