"Dissolved solids" refer to any minerals, salts, metals, cations or anions dissolved in water. This includes anything present in the water other than the pure water (H20) molecule and suspended solids. Suspended solids are any particles/substances that are neither dissolved nor settled in the water, such as wood pulp.
In general, the total dissolved solids concentration is the sum of the cations (positively charged) and anions (negatively charged) ions in the water. Parts per million (ppm) is the weight-to-weight ratio of any ion to water.
A TDS meter is based on the electrical conductivity (EC) of water. Pure H20 has virtually zero conductivity.
Conductivity is usually about 100 times the total cations or anions expressed as equivalents.
TDS is calculated by converting the EC by a factor of 0.5 to 1.0 times the EC, depending upon the levels.
Typically, the higher the level of EC, the higher the conversion factor to determine the TDS.
What is the best water to use?
The best water to use will have a low salt and contaminant level. The lower the EC/ppm, the greater level of control one has to the nutrients contained in the tank.
If the water used has a starting EC that is too high, you will be limited as to the amount of nutrient you can add to the system before reaching the highest recommended EC, resulting in a less than ideal nutrient solution which may lead to lower overall yieads.
Generally, if your tap water is good enough fo you to drink, it will be fine to use for your plants. Most hobby growers will use tap water without having too many issues, but for peace of mind test the water before adding the nutrients to it.
Keep in mind that pH will directly effect the accuracy of the EC reading, so it's best to ensure the EC levels are measured with a constant pH level.
Is it necessary to aerate the nutrient solution?
Aerating the nutrient solution is a good idea. It guards against stagnant water and improves plant health.