public float mass { get; set; }

The mass is given in arbitrary units, but the basic physical principles of mass apply. Newton's classic equation , demonstrates that the larger an object's mass, the more force it requires to accelerate it to a given velocity. Mass also affects momentum, which is significant during collisions; an object with large mass will be moved less by a collision than an object with lower mass.

The mass can also be automatically calculated when Rigidbody2D.useAutoMass is true. In this case, you cannot set the mass property directly.

Additional Resources: Rigidbody2D.useAutoMass.