Air density is the mass of air per unit of volume it occupies, and it is expressed in kilograms per cubic meter when using the metric system. In the free atmosphere, air density decreases when temperature increases or when the air is humid. However, air density increases when pressure increases.
Since air pressure decreases with increasing altitude, then air density is less at higher altitudes, and the available oxygen in the air is also less. This effect can be felt when climbing a mountain or when inside a plane going up. This difference in air density inside the airplane is not felt that much by passengers because of the pressurized air inside the plane.