Here is one way to understand air density.
Take a pint glass and fill it with Air. Would you then agree that in that pint glass you have maximum air density? You should agree as you cannot get more than one pint of air in a pint glass.
Pour half a pint water in to your pint glass of air. Water is heavier than air so it is going to push air out of your pint glass. So now you only have half the air in the pint glass. So would you now agree that the air in the glass is now less dense? You should because you have a 1 pint container with only half pint of air in it, which means you do not have maximum air density in the glass.
There are other properties that affect air density such as:
Temperature
Altitude
Maybe someone else will explain how those properties affect air density.