It's important to know that electric potential energies are measures in Joules (J), and electric charges are measured in Coulombs (C). Now, the electric potential is obtained by dividing the electric potential energy by the electric charge, which gives electric potential as a result expressed in Volts or J/C, that's what the unit J/C means.
Therefore, the voltage expressed in J/C refers to the electric potential energy, the ratio between Joules and Coulombs.