One is an optical guess at how much a sample scatters light. The other is a scale reading of how much solid material was actually filtered out and weighed. They usually move together — until the particles change shape, and they don't.
Both parameters exist to describe the same underlying thing — how much particulate material a water sample is carrying. But like BOD and COD, they answer that question through completely different mechanisms, and treating one as a stand-in for the other is a routine — and routinely wrong — shortcut. Turbidity is an optical measurement: a turbidimeter (nephelometer) shines a light through the sample and measures how much of that light gets scattered by suspended particles, reported in NTU (Nephelometric Turbidity Units). Total Suspended Solids (TSS) is a gravimetric measurement: a known volume of sample is physically filtered, the filter is dried, and the actual mass of solid material retained on it is weighed, reported in mg/L. One is a fast optical proxy. The other is a direct weight.