When nutrients dissolve in water, they split into ions. Each of these ions carry an electrical charge, which creates the potential for electricity to move through that solution. Pure water, on the other hand, is a poor conductor of electricity as it doesn't contain ions. The more ions a solution contains, the better it can conduct electricity.
EC is measured in milliSiemens per centimetre, but, unlike pH, there is no universal scale for measuring conductivity. In fact, there are at least four common scales in use, so it's always important to be aware of the scale your conductivity meter - and your nutrient supplier - is using.