From https://www.fdic.gov/bank/historical/reshandbook/glossary.pdf

A payment made to an uninsured depositor after a bank or thrift failure. The amount of the advance dividend represents the FDIC’s conservative estimate of the ultimate value of the receivership. Cash dividends equivalent to the board-approved advance dividend percentage (of total outstanding deposit claims) are paid to uninsured depositors, thereby giving them an immediate return of a portion of their uninsured deposit. Sometimes when it is projected that all depositor claims will be paid in full an advance dividend will be provided to unsecured creditors.