When you click on the Tweet Button it will create a Tweet with up to three elements in the Tweet:
1. The text that you want to Tweet. This is fully configurable to use any (or no) data from the current page. So you might choose to Tweet the document's name, or have a dedicated field for the text to be Tweeted.
2. The URL of the page that the Tweet button is on. In fact, this is shortened using Twitter's URL shortening service to give a URL starting with 'http://t.co'.
3. An optional 'via' screen name, which you can include so that people know the source of the Tweet's contents.
For further details and notes on configuring the Web Part, please see: http://www.appetere.com/Blogs/SteveM/January-2011/Tweet-button-web-part-for-kentico-CMS.aspx