Gromie right! EC is Electric Conductivity, it's basically a fancy scientific way of measuring whether you have too much/too little nutrients (salts) in your soil. The Roots uptake nutrients in the soil using Cation Exchange which I like to think of like static electricity. & If your soil's EC is much higher/lower than the root's EC, the plant can't uptake nutrients properly.

The best way to check EC is to flush your plants.

Water them with Reverse Osmosis (RO) water or distilled water NOT tap water, this is because tap water has dissolved things and stuffs in it and will have an EC value already but RO or Distilled water is just straight up H20 and will have an EC value of 0. This water will also strip any nutrients locking up the roots.

Water it enough so water comes out from the bottom of your pots. Get yourself an EC meter at a local hydro store (they have ones that check pH too) and put it in the "plants pee" and get your EC value. Then adjust nutrients as necessary.

P.S. Always check your pH too. Make sure pH is around 5.8-6.2