Pure water barely conducts; dissolved ions carry the current. So a conductivity reading in µS/cm is a stand-in for dissolved solids, and the translation is $\mathrm{TDS} = k \times \mathrm{EC}$ — read aloud: TDS equals k E-C. The factor k is empirical: nobody derived it, thousands of water samples voted on it, and the vote came back 0.55 to 0.70 depending on which salts dominate the water. Your first measured-not-derived constant — chemistry is full of them.

Two habits to keep. One: k is below 1, so the TDS estimate is always SMALLER than the meter number — if your answer grew, you divided. Two: this is an estimate wearing an honest label. Report it as "TDS by meter" and no chemist will object; call it exact and the lab that dried and weighed a real sample will happily correct you.