What is an EC Meter ? EC Meters are used to measure the nutrient solution for your plants. It determines the total amount of nutrient salts dissolved in the water. It cannot read or report anything about the individual nutrient components, only the total amount of all nutes in your solution.
When growing plants, they will require different “food”, nutrient or fertilizer strengths at different stages of their cycles. It is important to be giving the correct EC value as anything less or over could end up under feeding or over feeding and burning your plants. This also creates unnecessary stress for your plants and will slow down the growth of your plant.
Terminology used in the grow industry for EC Meters and testers :

EC = Electrical Conductivity (the measure of total dissolved solids in a solution)
CF = Conductive Factor (the measure of total dissolved solids in a solution)
ppm = Parts Per Million (the measure of total dissolved solids in a solution)
TDS = Total Dissolved Solids (read in ppm)
Truncheon = A type EC meter

Having an EC meter is most valuable for identifying danger zones with your nutrients early on, especially in hydroponic and aeroponic systems.