A millibar is a unit for measuring air pressure: how hard the mixture of gases around the Earth is pressing. Weather people use it on maps; a falling number often means a storm is coming. Milli- means one thousandth, so a millibar is one thousandth of a bar. In practice one millibar is the same size as one hectopascal (hPa), which is the name many weather services print now. So millibar is a slightly older name for the same everyday weather unit. You will see mb or mbar in older books and in aviation; you will see 百帕 on a Taiwan forecast. It is science English, not slang. It has nothing to do with a mill that grinds flour.

毫巴（數值等同百帕；台灣氣象多寫百帕）