TDS (total dissolved solids) is a measure of how much mineral content the water carries. It is reported in parts per million. The standard kitchen-purifier display reads it because it is cheap to measure: a conductivity probe and some math. The marketing around it has trained a generation of Indian homeowners to treat low TDS as the goal. It is one data point, not the goal.
What TDS includes
The dissolved solids in residential water are mostly minerals: calcium, magnesium, sodium, potassium, bicarbonates, sulphates, chlorides. Some of these are useful (calcium and magnesium are essential dietary minerals). Some are inert. Some, in excess, indicate other problems (high sulphate often correlates with industrial groundwater contamination).
TDS does not distinguish between these. A glass of water with 200 ppm of mostly calcium and magnesium (moderately hard mineral water) and a glass with 200 ppm of mostly sodium and chloride (slightly saline industrial groundwater) register identically on a TDS meter. They are very different waters.
What TDS does not include
TDS does not measure bacteria, viruses, pesticides, fluoride, lead, or any of the contaminants that actually matter for human health. A glass of water can read 50 on a TDS meter, look perfectly clean and still carry enough microbial contamination to make you ill. A glass at 800 ppm can be entirely safe to drink.
Aquaguard, Kent, and Eureka Forbes have built a marketing edifice on the TDS display. The display is useful, but only as one signal among several. Treating it as the headline measure has the consequence of teaching homeowners that an RO system stripping every mineral from the water, down to TDS 20, is the ideal. It is not. Below 80 ppm, the water tastes flat. Below 50 ppm, it is actively missing minerals the body uses.
Low TDS means dilute water, not pure water.
What to use instead
For deciding the right drinking-water treatment, TDS is a useful first cut: below 200 ppm, ultrafiltration with UV is sufficient; above 500, reverse osmosis is appropriate. Between those bounds, hardness, iron, and use case decide the answer. Beyond that initial sizing, what matters is the full water-test report: pH, hardness, iron, free residual chlorine and microbiological indicators, plus a fluoride screen where the region calls for it.
A reading on the TDS meter is a glance. A water test is the decision. Confusing the two is how homeowners end up paying twelve thousand rupees for a kitchen RO that strips minerals from water that did not need them stripped, while leaving the bathroom water entirely untreated, and that is the water actually breaking their hair and scaling their geyser.