Plain English Meaning
It is the standard air pressure at sea level that the world has agreed to use as a reference. When you set this number on your altimeter, every aircraft is measuring height from the same imaginary starting line in the sky.
Full Definition
The internationally agreed standard sea-level atmospheric pressure value, equivalent to 29.92 inches of mercury (inHg) or 1013.2 hectopascals (hPa). It is the baseline pressure used in the International Standard Atmosphere (ISA) and is the value pilots set on the altimeter when flying at or above the transition altitude to display pressure altitude (flight level).