We learn that gasses can contract or expand. When contracted, they may change into a liquid form and pressure is reduced. If the gas expands without being contained, there is little change in pressure, but if it is contained without dissipation, it will create pressure. The more it expands while being contained, the higher the pressure. Air is composed of gasses, but usually not being contained, we hardly notice the pressure, but it it is contained then we noticed the pressure. IE: an air compressor, inflated balloon, bicycle inner tube, etc.
Physics and chemistry