TThe density of the collider used to calculate its mass (when auto mass is enabled)
When using ::Rigidbody2D::useAutoMass, increasing the collider density increases its mass - as does increasing its area - as the collider mass is calculated as a product of density and area.
Note that by default, ::Rigidbody2D::useAutoMass is disabled, so the mass of the collider is set directly by ::Rigidbody2D::mass, and this density value is ignored.
See Also: ::Rigidbody2D::mass, ::Rigidbody2D::useAutoMass.