The density of air is its mass per unit volume. This means a volume of air at high density has a higher mass than the same volume at low density. And if there is a larger mass of air, it will contain proportionally more oxygen.
The density of air in the atmosphere changes at different temperatures, and altitudes. That means the air that enters an engine at different locations could have very different amounts of oxygen.
The amount of oxygen in air directly affects how well it supports combustion, so it can be important in determining an air-fuel ratio for an engine.
