What is air pressure?
Air pressure is the pressure exerted by the atmosphere ( the force you applied by air to compressed or unconfined).
the weight air puts on you at a given altitude
If you pump all the air out of a steel drum there's no air inside to hold it up allowing the outside air to push it inward, it collapses. I believe air pressure at sea level is around 1800 PSI. But I'm no sure. Suggest research.