The oxidation number represents the hypothetical charge an atom would possess if all its bonds were completely ionic. It plays a vital role in redox (reduction-oxidation) reactions, as it reveals the electron transfer occurring between different species.

In simpler terms, the oxidation number quantifies the number of electrons an atom either gains or loses when it forms a compound. The assignment of oxidation numbers to atoms within molecules or ions follows a specific set of rules. For instance, the oxidation number of an element in its elemental form is always $0$, while the oxidation number of oxygen in most compounds is typically $-2$.

Redox reactions are characterized by the transfer of electrons from one species to another. The species that loses electrons is referred to as being oxidized, whereas the species that gains electrons is said to be reduced. Throughout a redox reaction, the oxidation number of an atom changes, which signifies the movement of electrons.

For example, in the reaction between zinc and copper sulfate, zinc is oxidized to form zinc ions with an oxidation number of $+2$. Concurrently, copper ions are reduced to copper atoms with an oxidation number of $0$. The change in oxidation number of zinc from $0$ to $+2$ indicates the loss of two electrons, while the change in oxidation number of copper from $+2$ to $0$ signifies the gain of two electrons.

In summary, the oxidation number is a fundamental concept in redox reactions, as it illustrates the transfer of electrons between species. A thorough understanding of oxidation numbers is essential for predicting the outcomes of redox reactions and for balancing chemical equations effectively.