Atmospheric pressure is the weight of air pressing down on the Earth's surface. It is caused by the gravitational pull of the Earth on the air molecules in the atmosphere. This pressure decreases with altitude, meaning that higher elevations, like mountains, have lower atmospheric pressure compared to sea level. Measured in units like pascals or millibars, atmospheric pressure affects weather patterns and can influence activities such as flying and climbing. It plays a crucial role in the functioning of barometers, which are instruments used to measure this pressure and predict weather changes.