The contour lines ring the peak showing all the points that are at the same height, an equipotential line. On the hill potential is measured in feet of height.
Voltage is the measure of electric potential.
If two points are at the same potential or voltage a charge will have no tendency to move in that direction. There is no slope. If two points are at different potentials, positive charges will be pushed from the positive point toward the negative point. The greater the voltage difference or the closer the points are together, the greater the force. The strength or magnitude of this electric field is equal to the voltage difference between two nearby points divided by the distance between them.