Prepayment is early repayment of a loan by a borrower. In the case of a mortgage-backed security (MBS), prepayment is perceived as a risk, because mortgage debts are often paid off early in order to incur lower total interest payments through cheaper refinancing. The new financing may be cheaper because the borrower's credit rating has improved or because interest rates are lower, but in either case, the payments that would have been made to the MBS investor would be above market rates.
