Some biological staining procedures rely on the chemical process of oxidation and reduction.
Oxidation - A chemical reaction resulting in the removal of electrons from a molecule.
Reduction - A chemical reaction resulting in the addition of electrons to a molecule.
Stated simply, the oxidation-reduction reaction involves the transfer of electrons from one substance to another. Where staining techniques are concerned, oxidizers and reducers act on tissue so that dyes and/or reagents applied in subsequent steps are able to bind with the desired tissue elements.