pressure is the force exerted on you by the weight of tiny particles
of air (air molecules). Although air molecules are invisible,
they still have weight and take up space. Since there's a lot
of "empty" space between air molecules, air can be
compressed to fit in a smaller volume.
When it's compressed, air is said to be "under high pressure".
Air at sea level is what we're used to, in fact, we're so used
to it that we forget we're actually feeling air pressure all