What is a demand package, and how important is it to give one to the insurance company?

Once you finish treating, then your lawyer (or you if you are going it alone) will gather all the records and other documentation relating to your case and then create a demand package explaining why the other side is at fault and documenting all the injuries, medical care, lost wages, expenses, pain and suffering and other damages.  You can help this process along by informing your lawyer when you are finished treating and helping with your part of the documentation.