The potential difference of the electric field across the depletion region is the amount of voltage required to move electrons through the electric field.
This potential difference is called the barrier potential and is expressed in volts.
Stated another way, a certain amount of voltage equal to the barrier potential and with the proper polarity must be applied across a pn junction before electrons will begin to flow across the junction.
You will learn more about this when we discuss biasing in Chapter 2.