TDS (total dissolved solids) is a quick meter reading of everything dissolved in the water, used mainly by shrimp and soft-water breeders as a shortcut to track mineral content.
A TDS pen is cheap and gives an instant number, but it does not tell you what is dissolved — it lumps beneficial minerals in with nitrate and other buildup. It is most useful for spotting change over time in sensitive setups.
For most beginners, GH and KH tests are more informative than TDS; TDS becomes handy once you keep fussier species like Caridina shrimp.