What is TDS?

Total dissolved solids, a measure of all organic and inorganic substances that are dissolved within the tank water. Expressed in ppm (parts per million) usually.

Effectively we’re talking about contaminants in the water that isn’t water. So we’re talking about salts, minerals, metals, cations, and ions that can be present in water, including inorganic salts (e.g., calcium, magnesium, potassium, sodium, chlorides, carbonates and bicarbonates, and sulfates) and not to forget about the waste produced by our fish, shrimp, snails etc.
Why is TDS important?

Many fish and shrimp are able to accept a wide range of pH as a swing by 1 PH really isn’t a whole lot on the acidity scale, but many are more specific about the GH (general water hardness).

Animals that live in water are in balance with their surrounding water and are able to remain content with all the various salts and minerals (TDS, parts of which can be measured as KH or GH) within that water. More salts and minerals generally means higher GH (hard water, higher TDS) and less means lower GH (soft water, lower TDS).