A millibar (mb) is another measure of barometric pressure. It's actually preferred by meteorologists because it's more precise than inches of mercury. Millibars actually replaced an even more complicated measure, the newton, which was ridiculously hard to deal with. For example, standard barometric pressure at sea level is equal to a whopping 101,325 newtons, which is a pretty big number to manage! Enter the millibar, which is equal to one one-hundredth of a newton. Standard pressure in millibars equals 1013.25 and usually, you can knock off the decimal for most measurements, bringing the pressure value down to a much more easy to handle 1013.
The millibar is the measure of barometric pressure that's used on many types of weather charts and in computer model maps of the atmosphere.