The redox potential, usually abbreviated as Eh, is a measure for the oxidation or reduction state of a medium. You can measure it, and get a result in the scale of -1000 mV to +1000 mV. In soils, you commonly do not get lower than -600mV. High values are 400mV or above. These values show a potential for processes that then can occur; there is no direct link between a value and a specific process in nature.
A higher value shows that the medium is capable of oxidizing: there is a fast uptake of electrons possible. That means that microbes have oxygen available to break down organics, and that reducing processes occur less.
A lower value indicates reducing conditions. With low values, eg below 100mV, microbes have difficulty finding a fast electron receiver, and they need other pathways. You can also encounter chemical reduction, such as dissolution of P bound to Fe. At very low values, the chances for methane production, a very potent greenhouse gas, are high. There, organic material itself is used to store electrons.
More in depth information on the redox potential and its meaning can be found online, for instance at the wikipedia page: https://en.wikipedia.org/wiki/Reduction_potential and in several books. For wetland scientists, we advice to read the book "Wetlands", which gives a good overview of the connection between redox values and possible processes in a wetland.