Air pressure on any area is the amount of pressure that the air must exert to support the weight of the air directly above it. The lower you go, the more weight there is and so the more pressure there must be.
It works like this with anything that must support weight with its pressure -- air, water, the rock and iron that make up the interior of the Earth, etc. It's called hydrostatic equilibrium. See https://en.m.wikipedia.org/wiki/Hydrostatic_equilibrium.