If you borrow more than this, they hedge their bets by taking out an
insurance against you failing to pay the mortgage. They are the only ones to
benefit from the policy, but the bad news is that you have to pay the premium,
which is usually a single payment at the beginning of the mortgage.
Quite often this is added to the loan and you pay interest on it.
Some Lenders have decided to fund the risk themselves and do not charge a
premium, but may charge an interest surcharge at the beginning of the loan.