Let's say you take a 1-inch by 1-inch piece of wood that's 3 feet long, and let's say this piece of wood weighs 1 pound. If you were to stand that piece of wood on-end on your foot, it would place 1 pound of force on your toe. Since its cross-section is 1 square inch, it exerts 1 pound per square inch of force (1 psi) on your toe. If you were to take a 30-foot-long piece of the same wood and balance it on your foot, it would apply 10 psi of pressure. If it were 300 feet long, it would apply 100 psi, and so on.
Air works the same way. The atmosphere is about 50 miles "deep," and at sea level it exerts 14.7 psi. That is, a 1-inch-square column of air 50 miles high weighs 14.7 pounds. Our bodies think 14.7 psi of air pressure is completely normal.