Plain English Meaning
These are the on-board data files inside older GPS units that tell the receiver where every waypoint, airport, and approach is. The data has to be up to date for the unit to be legally used in instrument flying.
Full Definition
The internal navigation databases carried by older GPS receivers built to FAA Technical Standard Order C129 (TSO-C129). These databases store waypoints, navaids, airways, airports, and instrument procedures, and must be current and properly loaded for the receiver to be used for IFR navigation. TSO-C129 receivers are an earlier generation of GPS equipment that depend on receiver-based integrity monitoring rather than the -based used by later /C146 () units.