Hashtags relates to twitter and how to search what other twitter users are saying... Its a bit like typing a word into Google search but in this case you'll see other peoples tweets which use the same hashtag with the newest ones on top together with some stickies which are ad placements... A hashtag is basically the use of # preceding a word...
Obviously the numbers of people searching/using hash tags for a given word is different right across the board from millions to literally zero... There is a lot fluctuation with trends, current affairs, news etc...
You can use hashtags to funnel traffic back to your site.. People see your tweet and can then click on a link to your site... from their you obviously present your offer or encourage people to sign up to your list...