What is a biased source?

If a source has a bias, it means that the author of it had a personal interest in the subject they were writing about.  We will take the Bayeux tapestry as an example.  In 1066 there was a battle for the crown of England between Harold, and Englishman, and William, a Frenchman.  This was known as the Battle of Hastings.  The Bayeux tapestry is a French depiction of the events leading up to the battle, and the battle itself.  It clearly shows that William was the rightful king because it was a biased source (remember that William was French and the tapestry is French).  Any English source from the time would argue that Harold was the rightful king, because they would also be biased.