The barometer is used to measure air pressure and display weather tendencies. Basically, the higher you climb (ascent/terrain), the lower the barometric pressure.

Put simply:
An increase in air pressure usually signifies an improvement in the weather. If the air pressure drops, the weather is more likely to deteriorate. CASIO barometers can measure and display the current air pressure in the range of 260 hPA to 1000 hPA. Since the barometer takes regular measurements, the weather tendency can be "predicted" very well.