Reduction-oxidation (redox) reactions are chemical reactions in which reactants experience a change in oxidation number (which means these reactants either gain or lose electrons).

If an atom in a reactant gained electrons (its oxidation # decreased) it was reduced.

If an atom in a reactant lost electrons (its oxidation # increased) it was oxidized.

Since chemical reactions don't make or destroy electrons, oxidation and reduction must occur at the same time. As one reactant is oxidized, the electrons it loses are accepted by another reactant which is reduced.