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. Where staining techniques are concerned, oxidizers and reducers act on tissues so that dyes and or reagents applied in subsequent steps are able to bind with the desired tissue elements.