I had a hell of a time with panels 2 & 3; ultimately, the exposition didn't work, so I'll just infodump it here en masse.

Everything in the dreamworld is either a dreamer, an aux, or a construct. Auxes are objects dreamed up, but still attached to a dreamer, like Star's clothes and shoes. Constructs, hower, will stay built until actively edited by a dreamer; "mass" here is a metaphor, for a concept that means how much effort it took to dream up in the first place, and how much effort it'd take to edit. Constructs don't "naturally" move on their own, and their propulsion actually uses their "mass" as something akin to mass. Dreamers can naturally move as they will, and auxes naturally move with their dreamers; construct "mass" also applies if a dreamer moves the construct.

…But if there're other dreamers involved, that gets messy.