Plain English Meaning
Millibars are a way of measuring how much the air is pressing down. The higher the number, the higher the air pressure. Most of the world reports air pressure in millibars; the U.S. usually reports it in inches of mercury instead.
Full Definition
A millibar (mb) is a unit of atmospheric pressure equal to one-thousandth of a bar, or 100 pascals. Standard sea-level pressure is 1013.2 mb, which corresponds to 29.92 inches of mercury. Millibars (also called hectopascals, hPa) are the standard pressure unit used in aviation weather reports outside the United States and in surface analysis charts worldwide.