The percentage of dissolved coffee mass per gram of liquid in your brewed cup. 1.30 % TDS means 1.3 grams of coffee compounds are dissolved in every 100 grams of liquid. It's the number that quantifies what your tongue calls "strength."
Filter coffee normally lands between 1.20 % and 1.50 %. Below that the cup reads thin; above that, intense and viscous. Espresso, by comparison, sits at 8–12 %.
How it's measured
A digital refractometer drops a sample on a sensor and reads the refractive index of the liquid. Coffee compounds bend light differently from water; the device converts that bend into a TDS percentage. Most baristas use a VST or Atago model.
The reading takes about ten seconds. You then plug it into an extraction calculator — together with brew water and dose — to get a true extraction yield number, the percentage of bean mass that actually made it into the cup.
Why it matters
TDS turns "this tastes weak/strong" into a number you can chase across recipes. Note what it is and isn't: TDS is the strength side of the diagnosis only. Two cups at an identical 1.35 % can taste completely different if one is well extracted and the other isn't — which is exactly why the number is worth having, because it lets you rule strength out and go looking at extraction instead.
You don't need a refractometer to brew good coffee. You need it the moment you want to compare cups objectively, or dial in the same recipe across different equipment.