What a TDS reading tells you
TDS is an aggregate measure of dissolved material. Conductivity-based handheld meters estimate TDS indirectly, so readings are useful for screening and comparison but are not a substitute for laboratory identification of specific contaminants.
What it does not tell you
A low TDS reading does not by itself demonstrate that water is free from arsenic, microorganisms or other contaminants. Likewise, a high reading does not identify which dissolved substances are present.
TDS and RO
RO is commonly discussed where dissolved salts need to be reduced, but membrane design still requires broader feed-water information for pretreatment, scaling and fouling assessment.
Practical rule
Use TDS as one parameter inside a wider water-quality assessment, not as a single pass/fail test for water safety.