It’s just a loan, i.e. money that someone borrows from a bank, except that it is secured by real estate. So, for example, if you have a mortgage on your home but you fail to pay the interest that you owe each month, the bank can take ownership of your home. Since so many people use a mortgage to buy a home, mortgage rates (i.e. the interest rates on mortgages) have a huge impact on property prices.