Plain English Meaning
29.92 inches of mercury is the agreed-upon "standard" air pressure at sea level. Pilots and engineers use it as a fixed starting point so altimeters, charts, and aircraft performance numbers all refer to the same baseline.
Full Definition
The standard sea-level atmospheric pressure used as a reference in aviation, equal to 29.92 inches of mercury (also expressed as 1013.2 millibars or hectopascals). It is the pressure setting that defines the International Standard Atmosphere and is used as the baseline for altimeter calibration, performance charts, and above the .