A "balloon payment" is a once-off lump sum that is paid at the end of a car loan, covering off the outstanding balance. Because the lump sum is paid at the end of the loan you are only required to pay interest on part of the principle.
This allows you to repay only part of the principal of your loan over its term, reducing your monthly repayments in exchange for owing the lender a lump sum at the end of the loan term.