What is it?
Conductivity is water's ability to pass an electrical flow and basically means that there are more chemicals dissolved in the water (such as chloride, nitrate, sulfate, sodium, magnesium, calcium, or iron).
What does it mean?
Sometimes high conductivity is from natural sources (certain rockbeds create high readings, for example). But if an area acquires unusual readings, it could mean that that area is in trouble: too high or too low conductivity for an area mean local species cannot survive.
Acceptable levels of water conductivity for aquatic life.