The mass of the gas (in the air, oxygen, carbon dioxide, etc), a containment space and a force applied to it. The greater the force, and the smaller the container, the higher the pressure. For example, a syringe of normal air placed on a pressure sensor. As the syringe is closed, the space in which the gas can roam is decreased. Gas particles collide with one another more often in this state, and the pressure rises, vice versa to lower it.
Pretty sure about this, trying to remember back to chemistry class when I learned it.