In weather when air pressure decreases warm air rises.
As the warm air rises as a result of the decrease in air pressure the warm air then cools as it goes higher into the atmosphere.
When the air pressure in weather decreases and gets lower it's usually a good indicator of a storm approaching and you can expect storms and rain or at least rain and cloud days.
When the air pressure increases in weather it is a good indicator that the weather is gonna be sunny or at least nice without rain or storms.
So decrease in air pressure means that storms are approaching and low pressure in the weather means that nice weather and sunny days are ahead.