Mass is a characteristic of a physical bodies which described the quantity of matter contained in those bodies.
Mass was an important measurement to understand Changeling physiology. Odo was under significant study in his early life, and Julian Bashir noted that Odo's mass was similar whenever he was in humanoid form, and changed when he changed shape. When he suffered a poison from his people in 2372, Bashir first noted that Odo's mass fluctuated rapidly. (DS9: "Broken Link")