EC stands for Electric Conductivity (mS / liter) and with that we measure the amount of (usable and unusable) salts in water. The EC meter simply measures the amount of electrical resistance and that also explains why a medium like earth? Wet? must be if one measures it.
An EC meter measures a total sum of all food salts. An EC of 0.1 / 100L from one brand of nutrition therefore does not have to have the same nutritional value as an EC of 0.1 / 100L of the other brand.
Most cheaper plant foods contain many elements such as chloride, cadmium and other dirt. The EC meter also measures these salts.
Growers who grow their crops by weight also try to get the plant to absorb as many salts (building materials) as possible for a heavier end result.
These salts must be available and absorbable and preferably balanced in such a way that it stimulates each other to be absorbed.
The best way to measure this in hydro or nft systems is by withdrawing some water from the medium with a syringe and then measuring it. For soil or coco, it is best to mix the mediums lightly with distilled water in a cup and measure after soaking. It is also possible with normal water, but is of course less accurate.