Any gas has certain properties that we can detect with our senses. The values and relations of the properties define the state of the gas. The pressure of a gas equals the force exerted by the gas divided by the surface area on which the force is exerted. The temperature of a gas is a measure of how hot or cold the gas is. Gas is composed of a large number of molecules, and the sum of the mass of all the molecules is equal to the mass of the gas.
Gas occupies some volume in three dimensional space.
For a given pressure and temperature, the volume depends directly on the amount of gas. Since the mass and volume are directly related, we can express both the mass and volume by a single variable. When a gas is moving, it is more convenient to use the density of a gas, which is the mass divided by the volume the gas occupies. Either variable can be used to define the state of the gas, since they are reciprocals. The state of a gas can be changed by external processes, and the reaction of the gas can be predicted using the laws of thermodynamics.