Mortgage protection is life cover taken out for the term of your mortgage. If you, or the person you have the mortgage with, dies, it pays off the loan. The most common type is decreasing cover: the amount insured falls as the mortgage falls, while the premium stays the same.
Your lender is legally required to make sure you have it before giving you a mortgage on your home. There are exceptions, for example if you are over 50, the loan is not on your home, you already hold enough life cover, or you cannot get cover because of your health or your job.