What is meant by electron transfer potential?
the electron transfer potential is based on the difference in the affinity of elements/molecules to bind electrons. the affinity of the element derives from it's components as shown in the elements table, just like all other characteristics. for example, oxygen has higher affinity to bind new electron than carbon. the affinity of the molecule derives from the atoms that build it, their electric potential and the connections between those atoms.

this potential between the two elements/molecules (lets say: NADH, O2, iron–sulfur clusters) holds energy that can start a reaction which will never start otherwise.

I have tried to keep it simple. for more reading about oxidation-reduction ( reactions involve the transfer of electrons between chemical species) read TERMODINAMICS AND CHEMICAL KINETICS p.255 as suggested by @Roland.