Impedance is a measure of how much resistance is present in an electric fencing system. High impedance energizers work best when resistance is higher from dry grass touching the electric wires, when the soil is very dry, or when the fencing system could benefit from additional ground rods.
Low impedance energizers work best when resistance is low, such as during times of the year when the soil is moist, the grass is green, and there are plenty of ground rods installed. Some energizers are designed to withstand both conditions and are referred to as wide impedance energizers. They tend to combine medium to high voltage with much greater current levels.