Good question, mass in Blender refers to the scientific term which means the object's resistance to acceleration. Similar to density but not quite the same: https://en.wikipedia.org/wiki/Mass
Really thanks! As I'm not good in English, when I used google to translate 'mass', the answer confused me. While I saw the equation 'F = ma', I understand what that means, since the equation expression of Newtonian physics is spelled the same way around the world :)
