It is an obligation (法律上的承諾) given by a 3rd party ( the guarantor 擔保人 ) to the bank to pay off the money lent to a person should that person( Borrower 借款人) is unable to pay off the debt i.e. default in payment. If the borrower can pay off some part of the loan e.g. $20 out from $100, the bank will demand the guarantor to pay off the remaining balance $80 plus accrued interest (累積利息).