It is the affinity of a substance to accept electrons i.e. it is the
potential for a substance to become reduced.
Hydrogen has the lowest redox potential (-0.42 volt), while
oxygen has the highest redox potential (+0.82 volt). The redox
potentials of all other substances lie between that of hydrogen and
oxygen.
Electrons are transferred from substances with low redox
potential to substances with higher redox potential. This transfer
of electrons is an energy yielding process and the amount of
energy liberated depends on the redox potential difference
between the electron donor and acceptor.