A TDS Meter indicates the Total Dissolved Solids (TDS) of a solution, i.e. the concentration of dissolved solids in it. Since dissolved ionized solids such as salts and minerals increase the conductivity of a solution, a TDS meter measures the conductivity of the solution and estimates the TDS from that.
Dissolved organic solids such as sugar and microscopic solid particles such as colloids, do not significantly affect the conductivity of a solution so a TDS meter does not include them in its reading.
A TDS meter typically displays the TDS in parts per million (ppm). For example, a TDS reading of 1 ppm would indicate there is 1 milligram of dissolved solids in each kilogram of water.