A an electric potential (Volt) is equivalent to the unit of J/C. This is because if you move a charge through a potential you gain energy. Volt is kind of a mysterious unit and by the time people reach it the sub-units of J and C are really well understood so it often helps to make sense out of potential. In the same way you can understand speed by thinking of it as distance/time.
The standard reduction potential can be thought of as the potential produced if the reaction happens (if positive) or required to make it happen (if negative). It is determined usually by measuring the total potential (of both reactions) with one half reaction being the one you are interested in and the other which has a zero potential so the standard reduction you are looking for is then just the net potential.
When you have two of those reactions which are both non-zero, the net potential does depend on both.
There is an excellent article on it at CLT :
Standard Reduction Potential