A barometer is a device used in meteorology for measuring atmospheric pressure, generally to determine altitude or weather changes. A rising barometer signifies increasing air pressure while a falling barometer indicates decreasing air pressure.
Air pressure is different at different points on the planet and changes over time. In space, there is a nearly complete vacuum, so the air pressure is zero. On Earth, there are many miles of air molecules wielding pressure due to the force of gravity. Similarly, on any given day, air over a desert would be expected to have lower pressure than the air over an ice top.