What does this equation mean?

The remaining question is what the electrical quantities are referred to. The current is obtained from a voltage drop across a stable resistor; both voltages are measured against the Josephson constant, taken as KJ = 2e/h, and the resistance against the von Klitzing constant, taken as RK = h/e², so that KJ²RK = 4/h and the mass reduces to h multiplied by an experimental frequency term and divided by the product of local gravity and coil velocity [ 4 ] : $K_{\mathrm J} = \frac{2e}{h}, \qquad R_{\mathrm K} = \frac{h}{e^{2}}, \qquad K_{\mathrm J}^{2} R_{\mathrm K} = \frac{4}{h}$. This is where the quantum electrical standards earn their place. Since 1990 the practical realization of the volt and the ohm had rested on the Josephson and quantum Hall effects, but with conventional values assigned to the two constants — a decision Taylor and Witt set out when the new reference standards took effect on 1 January 1990, intended to “improve significantly the international uniformity of electrical measurements and their consistency with the SI” [ 9 ] . The consequence was a parallel electrical unit system, close to the SI but not identical to it [ 3 ] . Fixing h and e together dissolved the parallel system: KJ and RK are now calculated exactly from the defining constants, and the conventional values were retired [ 3 ] . A Kibble balance is therefore not merely an accurate balance. It is a mechanical quantity expressed entirely in quantum-referenced electrical terms, which is why the electrical metrology community wanted the kilogram and the ampere redefined in the same act.