Mass of the rigidbody.
The mass is given in arbitrary units but the basic physical principles of mass apply. From Newton's classic equation force = mass x accelelation, it is apparent that the larger an object's mass, the more force it requires to accelerate it to a given velocity. Also, mass 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 useAutoMass is true. In this case, you cannot set the mass property directly.