Iso lines are lines on which a field has constant value. The most common examples are height lines on maps, where the iso lines are called height lines. How close they are together is an indicator of steepness, and going down the steepest direction one would travel perpendicular to the height lines.
Lucky us, they also make for pretty pictures and contour plots. Here is an assortment of a few random hills (Coulomb potentials, if you’re familiar with electrodynamics), but they might as well be actual hills on an actual map.
