Conductivity (measured in ppm, EC, TDS, CF) is the total soluble salts contained within a liquid solution.
Pure water does not contain salts, so it has a conductivity measurement of zero. By adding soluble elements(fertilizers or other additives) to the water, electricity is able to move through the solution giving it a conductivity rating. This gives you a relative strength of the solution you are giving to your plants.
Controlling this nutrient strength is all important in providing the best conditions for your crop. Without proper measurement your crop could fail from root burn brought on by too high a nutrient level, or death by natural causes from not receiving the vital elements they need to grow.
Different crops grow well at different levels of nutrient strength. It is important to control the nutrient strength to provide the conditions in the root zone which provide maximum uptake of balanced nutrition into the plant, while minimising stress.
For each crop there is a range in which the nutrient strength is optimal, providing enough nutrient for growth, but not so strong that the concentration of salts in the plant gets too high. Electronic meters are available to measure the conductivity of the nutrient solution.