What does this equation mean?

The structure is the same one that makes an Aharonov–Bohm phase or a Berry-phase holonomy nonzero: a closed path in a classical parameter space picks up a phase set by a curvature that lives on that space, even though nothing classical distinguishes the endpoint from the start. Here the “curvature” is the commutator [$K_i$,$P_j$] itself, and the “charge” that couples to it is mass. The analogy is a guide to intuition, not a claim of identity — there is no background field on the group manifold in the electromagnetic sense, only the algebra’s own structure constants — but it explains why the effect survives however the loop’s interior path is traced, so long as the net translation and net boost at the end match: like a Wilson loop, the phase depends on the loop’s declared endpoints in group-parameter space, not on the particular way the apparatus was walked between them.