atmospheric pressure
Atmospheric pressure is the pressure exerted by the whole column of air above you, pressing in from every side. Although air feels weightless, the layer of atmosphere reaching tens of kilometres up is heavy, and it pushes on your body, on the ground, and on everything around with a steady squeeze. It answers the question: how hard does the ocean of air we live at the bottom of press on us?
Precisely, atmospheric pressure at a point is the weight of the air above a unit of area, transmitted through the air as a fluid. At sea level its average value is about 101 325 Pa, which is defined as one standard atmosphere (1 atm), roughly 101 kPa or 760 millimetres of mercury in a barometer. Because it comes from the weight of air overhead, atmospheric pressure falls as you climb: on a high mountain there is less air above you, so the pressure and the oxygen both drop.
We do not normally feel this enormous push because it acts equally in all directions and is balanced by the pressure inside our bodies and fluids. Its reality shows up dramatically when the balance is removed: suck the air from a sealed can and the outside atmosphere crushes it flat. One honest note about scale, atmospheric pressure is equivalent to the pressure at the bottom of about 10 metres of water, which is why a simple suction pump cannot lift water higher than roughly that.
The atmosphere presses about 101 000 Pa on every surface. On a door of area 2 m^2 that is a force of F = P × A = 101 000 × 2 ≈ 200 000 N on each face, but because the air pushes equally on both sides, the two forces cancel and the door swings freely.
The air presses hard on everything, but equally from both sides, so we hardly notice it.
We do not feel atmospheric pressure because it pushes equally from all sides and is balanced from within; it becomes obvious only when that balance is broken, as when a can is evacuated and collapses.