If you travel by air, pressurization inside the airplane lets you breathe normally at high altitudes, where the air is thin. Pressurization keeps the density of oxygen and other gases in the cabin's air similar to conditions on the ground.
Human bodies function best on Earth, where the atmosphere has the right amount of air pressure and oxygen for people to breathe. Pressurization of an airplane's cabin involves pumping in and circulating conditioned air to keep it safe and comfortable even at 40,000 feet above the ground — and even in outer space! People are able to live and work aboard spacecraft and space stations thanks to pressurization systems that allow astronauts to breathe even where there is almost no air outside.