When you give someone money that you owe them, you repay them. A bank may loan you money to buy a new car, but you'll have to repay the money, plus interest.
You might repay a bank loan, or repay your brother the dollar you borrowed last week. You can also repay things other than money: "How will I ever repay your kindness and support?" Repay comes from the French repaier, with its "back" prefix re- and payer, "to pay."