Some biological staining procedures rely on the chemical process of oxidation and reduction.
Oxidation: The chemical reaction resulting in the removal of electrons from a molecule.
Reduction: The chemical reaction resulting in the addition of electrons to a molecule.
In simple terms, the oxidation-reduction reaction involves the transfer of electrons from one substance to another. In staining techniques, oxidizers and reducers act on tissues so that dyes and/or reagents applied in subsequent steps can bind with the desired tissue elements.