a sub-prime mortage is when the persons credit rating isn't good the bank stiil gives them a loan, sub-prime loans have lower down payments than usual, meaning equity building in the house will not be as high
http://www.usatoday.com/money/perfi/columnist/waggon/2007-03-15-subprime-woes_N.htm

(http://www.investopedia.com/terms/c/creditdefaultswap.asp answer)

secures your credit (mortge) if you loose a job or something happens to you and your credit insurance will cover a percentage of your coverage
http://en.wikipedia.org/wiki/Credit_insurance