Plain English Meaning
It is the standard air pressure used worldwide as a baseline. When set in the altimeter, every aircraft is measuring height from the same imaginary reference, so flight levels match between aircraft.
Full Definition
The international standard sea level atmospheric pressure value, equal to 29.92 inches of mercury or 1013.2 millibars. It is the pressure setting pilots place in the altimeter's Kollsman window when flying at or above the transition altitude in regions that use hectopascals, so that all aircraft display pressure altitude (flight levels) on a common reference.