In aqueous solutions, redox potential, or oxidation-reduction potential (ORP) measures the solution's tendency to either gain or lose electrons when it is subjected to change by introducing a new species.
A solution with a higher (more positive) reduction potential than the new species will tend to gain electrons from the new species (i.e., to be reduced by oxidizing the new species), and a solution with a lower (more negative) reduction potential will tend to lose electrons to the new species (i.e., to be oxidized by reducing the new species).