How do you spell millibars and what does it mean?

That is the correct spelling of millibar (mb) and it means "1/1000th of a bar" which is a non-SI unit of pressure, chiefly atmospheric pressure. A bar is defined as 100,000 pascals (100 kilopascals) so that "standard pressure" on Earth is 1.01325 bars, equal to 101.325 millibars.