As a setting, a default is automatic. You weren't sure why your new TV kept returning to the factory defaults until you realized you were sitting on the remote.
Default can also refer to a lack of other options. You shop at Green's by default; it's the only grocery store in town. Default is also a failure to pay a financial obligation. The number of loan defaults was down this month. As a verb, it means "to fail to pay." Your credit score will go down if you default on a loan.