What does this equation mean?

Because m enters only through the fixed central charge and $\mathbf b$,$\mathbf v$ are the loop’s own declared parameters, $m_{\rm op}$[$\mathcal L_{\rm G}$] does not depend on which inertial frame is used to describe the apparatus from outside: boosting the whole laboratory changes how an external observer labels positions and velocities, but it does not change which group elements were composed in the lab, nor the central charge attached to the representation. The construction is a scalar of the loop, not an artifact of a chosen external frame. A torsion balance testing the ordinary, classical equivalence principle never needs this fact, because nothing about a suspended mass on a fiber depends on whether translations and boosts secretly fail to commute at the operator level; that failure is invisible to any apparatus that never asks a quantum phase to remember a loop.