This refers to the level of oxygen present in the air at that altitude with the temperature at 60 degrees F, with a standard pressure of 29.92, and dry air, or zero percent humidity.

Surface chart of the cyclone that resulted in the lowest barometric pressure ever measured in the United Kingdom; 925.6 mb at Ochtertyre in Scotland on January 26, 1884.

Many different factors affect the density of air. Most measurably, as altitude increases, air becomes less dense, decreasing atmospheric pressure.