Gases exert pressure by applying a force when they collide with a surface. That surface can be the interior walls of an scuba tank or your skin. Remember we are surrounded by gases all the time. We breathe the oxygen, and add to the carbon dioxide when we exhale. The atmosphere is composed of many gases, but it is primarily nitrogen (~79%), with oxygen (~19%), and very little quantities of many other gases including carbon dioxide, argon, nitrogen compounds, unburned hydrocarbons, ozone, ...
The atmosphere is composed of gas particles from high above the earth to here on the surface. All of them moving and colliding. Here at the surface, we actually feel the "weight" of all those gas particles from the top of our head all the way to outer-space. This is called atmospheric pressure. The normal pressure exerted by the atmosphere at sea level is 1 atmosphere (atm).
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