A potentiometric surface is based on hydraulic principles. For example, we know that two connected storage tanks with one full and one empty will gradually fill/drain to the same level. This is because of atmospheric pressure and gravity. A potentiometric surface is the imaginary line where a given reservoir of fluid will "equalize out to" if allowed to flow. This idea is heavily used in city water supplies - a tall tower containing the water supply has a great enough potentiometric surface to provide flowing water at a decent pressure to the houses it supplies.
In groundwater "potentiometric surface" is a synonym of "piezometric surface" which is an imaginary surface that defines the level to which water in a confined aquifer would rise were it completely pierced with wells.[1]