Barometer is an equipment to measure atmospheric pressure, and millibar is a unit used for measuring atmospheric pressure.
The millibar is derived from another unit of measuring atmospheric pressure, bar, which is equal to equal to atmospheric pressure under standard conditions. Among others, these standard conditions include pressure at mean sea level and atmospheric temperature of 0 degree centigrade. 1 millibar is equal to one thousandth of 1 bar.
Another unit used for measuring atmospheric pressure is height of column of mercury. In this unit the atmospheric pressure at sea level is about 760 millimeters of mercury, which is equal to 1.013 millibars.