The atmospheric pressure decreases by increase in height or altitude.
Thus pressure high up in the sky is very low. Thus airplanes require to maintain its cabin pressure (pressure inside the plane).
At the ground, the pressure inside our body will cancel out the atmospheric pressure. But high up in the sky, due to low pressure, our bodies will expand which is fatal for us. Furthurmore, we will not get enough air and oxygen due to the same low pressure.
Thus pressure inside the plane is maintained by pumping in air and this system is called Cabin Pressurization System. Due to this system, the passengers will feel the same pressure when they are in the ground.