Understanding foam density
Density measures material, not feel. A 1.8-density foam weighs 1.8 pounds per cubic foot. More material in the same space means more of the structure that holds a cushion's shape, which is why density is the number to look at when the question is how a cushion will hold up under daily use.
Firmness is a separate axis with its own scale. A dense foam can be built soft and a lighter foam can be built firm, so read the two together: density speaks to longevity, firmness to how the seat feels when you sit down.