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The translocation of electrons from organic fuel molecules releases energy stored in these molecules, and this energy is ultimately used to produce ATP or adenosine triphosphate molecule.

Redox reactions are those in which there is the transfer of electrons at one or more places, in a reaction, from one reactant to another.

Both of these phenomena can be seen in the redox reactions. In a redox reaction, the loss of electrons from one reactant is referred to as oxidation. The addition of electrons to other substances is known as reduction.

The addition of negatively charged electrons to an atom reduces the amount of positive charge in it. The substance which gives out the electrons is known as a reducing agent, and the substance which accepts the electrons is an oxidizing agent.

It is to be noted that there may be an incomplete transfer of electrons from one substance to another; some change the degree of electron sharing in covalent bonds.

In the given reaction, $C_4{H}_6{O}_5+NA{D}^{+}→C_4{H}_4{O}_5+NADH+H^{+}$,

$C_4{H}_6{O}_5$ is oxidized as it donates the electrons and, thus, acts as the reducing agent. $NA{D}^{+}$ in this case gets reduced as it accepts electrons, which leads to a decrease in the positive charge present on it, and it will form NADH; $NA{D}^{+}$ acts as the oxidizing agent.