The answer is more straight forward than people realize.
A mortgage is simply a loan you take out from a mortgage lender to pay for a property. The property is used as security for the mortgage.
If you don't pay back the loan, as per the agreement you make, then the mortgage lender can take possession of the property and sell it to repay the loan. Lenders realize that your home is excellent security therefore offer very attractive interest rates to people wanting a mortgage.
The loan is divided into the capital which is the amount of money you borrowed to buy your property and the interest which is the amount the mortgage lender charges for lending you the money.