Turbidity is the relative transparency of a fluid, in this case, surface water. The transparency depends on the amount of suspended particles in the water, so the measurement of turbidity is closely linked to total suspended solids. The particles suspended in the water can be almost anything, from silt to precipitated chemicals.
When surface water is turbid, less light enters the water, and aquatic plants cannot perform photosynthesis and less organisms can survive, thus lowering the productivity of the ecosystem. Turbidity is also linked with low dissolved oxygen, colder temperatures, harmed gill function, and degraded spawning areas. Fine particles can kill fish directly, as well.