Mass is a property of objects proportional to the amount of force required to change their momentum (or, how heavy they are). There is no way to truly quantify mass in Besiege in real-world units; the density of blocks is both inconsistent and unaffected by scale. It is possible for a human-sized puppet, a plane, and a battleship to have the same mass.
Apart from that, mass obeys real-world physical laws. Mostly.
Most objects in Besiege have a set mass (most of which can be derived using mods, or found on their wiki pages). A handful of blocks and objects have adjustable mass, such as Ballast, Scaling Blocks, and Primitives.