Plain English Meaning
It is a way of measuring air pressure. The number tells you how tall a column of liquid mercury the air pressure could push up in a sealed tube, measured in inches.
Full Definition
A unit of pressure measurement equal to the pressure exerted by a column of mercury one inch tall at standard temperature and gravity. In aviation, it is the standard unit used for altimeter settings and manifold pressure in piston engines. The international standard sea-level atmospheric pressure is 29.92 in. Hg.
Context Anchor
You may see inches of mercury during a before-takeoff check when setting the altimeter or checking engine instruments.