An isopotential map is a map that shows different areas that have the same amount of electricity flowing through them.
Let's say you have a battery and you attach wires to the positive and negative ends of the battery. The electricity from the battery flows through the wires and creates a path of electricity. This path of electricity is called an electrical field.
Now, if you put a lot of measuring instruments (like volt meters or ammeters) in different points of this electrical field, you can measure the amount of electricity flowing through each of those points.
An isopotential map takes all those measurements and creates a map where the areas that have the same amount of electricity flowing through them are colored the same. For example, if you measured the amount of electricity in the middle of the electrical field and you found out that it's the same as the amount of electricity at the bottom left side, those two areas would be colored the same in the isopotential map.
This type of map is really useful for scientists who work with electricity, because they can visualize how the electricity is distributed in their experiments or in a circuit so they can make adjustments if they need to.