With mortgage assumption, you take over, or assume, the seller's mortgage on the purchased property. You accept full responsibility to pay the mortgage according to the existing mortgage terms. You need the lender's approval before you can assume the seller's mortgage.
As the buyer, mortgage assumption may be a good option for you if market interest rates are higher than the interest rate in the seller's mortgage on the closing date.
Mortgage assumption may be a good option for the seller if they're selling their home before the mortgage maturity date and not getting a mortgage on a new property. Mortgage assumption helps the seller avoid prepayment charges.