Oxidation-reduction (Redox) reactions involve the transfer of electrons from one substance to another. Redox reactions must occur together (in couples). One compound must lose electrons and the other gain electrons.
The substance which loses electrons is called the reducing agent while the substance which gains electrons is called the oxidizing agent. The reduction potential is the measure of the ability of one compound to reduce another.
For example, in the following reaction O2 is the oxidizing agent (and is reduced) and NADH is the reducing agent (and is oxidized).
1/2 O2 + NADH + H+
|H2O + NAD+|