A gas will exert pressure on its container by continuously trying to expand its area, which pushes gas molecules against the container's walls. A gas with higher energy will exert more pressure than one with lower energy, and a container with more gas molecules with receive more pressure than one with less molecules.
The pressure of a gas is one of its main properties. It is generally measured in units that show the force of the gas divided by the area in which it is being exerted.
One example of a container that experiences gas pressure is the Earth's atmosphere. Gas is continuously pushing against the sites of the atmosphere, which causes people to experience a pressure of 15 pounds per square inch. However, people do not feel this due to the counterbalance of the pressure in their bodies.