Conductivity, along with pH, is an important factor for determining the characteristics of aqueous solutions.
For our purposes, it is a measure of a liquid's ability to conduct an electric current. Liquids, and in particular water, dissolve various substances, and in many cases, when a substance dissolves in water, the latter conducts electric current more easily than before. Therefore, in simple terms, conductivity can be said to be a quantity that indicates the amount of substance dissolved in a liquid.
Note: Some substances, however—such as sugar—do not turn into ions when they dissolve in water, and therefore do not significantly affect the conductivity of the solution.
As an example, let us take natural water, the liquid with which we are most familiar. Seawater has a very high conductivity because it contains a large amount of dissolved salt. In contrast, rain, river and lake water—in their original state—contain only small amounts of dissolved substances, so that they have a very low conductivity. When water of low conductivity is polluted, its conductivity increases. For this reason, conductivity is also used as an indicator of pollution in these types of water.