It is either the oxidation or the reduction reaction component.
Example from the standard Galvanic reaction.
Full equation.
Zn(s) + CuSO4(aq) --> ZnSO4(aq) + Cu(s)
Oxidation.
Zn(s) --> Zn^2+ + 2e^-
Reduction.
Cu^2+ + 2e^- --> Cu(s)
What are redox half-reactions?