In simple terms, density is the mass of anything divided by the volume it occupies.
Scientists usually measure air density in kilograms per cubic meter.
At sea level, if the air is completely dry, and the temperature is 0 degrees Celsius, a cubic meter will have 1.275 kilograms of air in it. In other words, the density is 1.275 kilograms per cubic meter. The air's density depends on its temperature, its pressure and how much water vapor is in the air. We'll talk about dry air first, which means we'll be concerned only with temperature and pressure. The molecules of nitrogen, oxygen and other gases that make up air are moving around at incredible speeds, colliding with each other and all other objects.