Suppose we have a voltic cell. In one half cell, we have a zinc electrode (or any other) and in the other we have a hydrogen electrode (or any other).
Now, if I say that the electrode potential of zinc is 5 volts, would it mean that it (5 volts) is actually the potential difference between the two electrodes i.e zinc and hydrogen?
The actual potential of any object, including an electrode cannot be determined. Therefore, to maintain uniformity and to provide relative values of electrode potentials to different conductor-electrolyte pairs, IUPAC passed a convention according to which, the standard electrode potential is the potential difference of the cell (potential of cathode minus that of the anode) formed by using the concerned redox pair as the reduction(cathode) half cell and a Standard Hydrogen Electrode as the oxidation half cell. Therefore the standard electrode potential values you get is actually the potential difference of that redox pair acting as cathode and SHE as the anode. Therefore, in any voltaic cell, The actual potential difference between electrode potentials of the cathode and the anode ($E_c-E_a=E$) is the actual potential difference or the EMF(for zero current) of the cell. In case there is current drawn from the cell, the actual potential difference between the electrodes will be reduced on account of the internal resistance of the electrolyte and will be lower than the EMF. (In reality, Overpotentials and liquid junction potentials further complicate the analysis, but by using a salt bridge they can be minimised). Moreover in case the concentration (activity) of various involved species are not unity than you have to use the Nernst Equation to find the electrode potential and use it instead of the standard potentials.
One simple rule to remember is that the net emf of any voltaic cell is the difference between the standard electrode potential of cathode minus that of the anode.
