The story of Tobit is found in what is called the Apocrypha, that is, a collection of books written very much in the manner of the Bible, and formerly bound in Bibles between the Old and the New Testament.
The story goes that when Enemessar, King of Assyria, conquered the people of Israel, he led away many of them captive into Assyria, among them the family of Tobit, his wife Anna, and their son Tobias. They settled in Nineveh, and Tobit, being an honest man, was made purveyor to the king. That is, it was his business to provide food for the king's household.