Atmospheric pressure refers to the weight of air that is pressing down on a particular surface. It is often measured in units called millibars or inches of mercury, and it can vary depending on factors such as altitude, temperature, humidity, and weather conditions. At sea level, atmospheric pressure typically ranges from 760 to 1030 millibars (or 29.5 to 30.5 inches of mercury), but it can fluctuate throughout the day and during different seasons or weather events.