What exactly is 'delinquent tax liability' and why is Indiana keeping my state refund?

I got a letter from the state of Indiana saying that they were keeping my state refund as part of a payment toward a delinquent tax liability for 2006. There was no explanation of what this was so I have no clue why they would be keeping my refund. I can call on Monday (of course there's no web access to try and figure out what it means) but I just wondered if anyone had any insight on this?