Many people cannot afford to buy their home outright. A mortgage is a loan secured against a property that funds the part of it a borrower cannot afford themselves.
For example if you have a deposit of £50,000 towards a property that costs £200,000, your mortgage will be for the difference of £150,000. You pay the lender interest (alongside capital under a repayment mortgage) at a rate that varies according to several criteria including the size of the loan and whether you want to agree a fixed rate or a variable rate. Should you fail to make monthly payments on time the lender has the right to take possession of the property.