Should the total rigid-body mass be automatically calculated from the Collider2D.density]] of attached colliders?
When false, the explicitly set mass is used for the rigid-body mass. When true, the mass is automatically calculated from all attached Collider2D as a product of their Collider2D.density]] and area.
When true, inside the Unity editor, the Collider2D.density]] property will appear on any attached Collider2D and the mass property will become read-only.
When false, the mass property can be written to and the Collider2D.density]] property is not shown.
Additional resources: mass, ::Collider2D.density.