Deflocculation is the process of making a clay slip flow as a thin, pourable liquid using a small amount of chemical additive, rather than by adding enough plain water to thin it, which would require far more water than deflocculation does.
Clay particles carry a slight electrical surface charge and naturally clump together, or flocculate, in suspension; that's why plain water mixed into dry clay makes a thick, sluggish mud rather than a fluid liquid, even at fairly high water content. A small addition of a deflocculant, an electrolyte such as sodium silicate or soda ash, or a proprietary version like Darvan, neutralizes or reverses that surface charge, so the particles repel each other slightly instead of clumping together. The result flows freely at a much lower water content than plain water alone could ever achieve.
A properly deflocculated casting slip (see slip casting) needs far less water than an ordinary throwing or handbuilding clay consistency, which matters directly for casting: less water in the slip means less water has to leave the piece again as it dries, which means less shrinkage and fewer drying cracks in cast work. Adding too much deflocculant overdeflocculates a slip, thinning it further at first but eventually making it thick and gelled again past a certain point, which is why potters test a new batch incrementally rather than adding a deflocculant all at once.
A deflocculated slip's consistency is usually checked with a hydrometer or by weighing a fixed volume, the same specific gravity measurement used for dipping and pouring glazes, since "how thin is it actually" isn't reliable to judge by eye alone. A casting slip that is well deflocculated but too concentrated still pours poorly, so getting a good casting consistency is a balance of deflocculant amount and overall water content together, not deflocculant alone.
Specific gravity alone doesn't capture how the slip actually flows, since two batches at the same specific gravity can pour very differently depending on how well-deflocculated each one is. A second test, viscosity (commonly measured with a Ford cup, timing how long a fixed volume takes to drain through a small hole), tracks that separately. Plotting viscosity against deflocculant amount traces a deflocculation curve: viscosity drops steeply at first as deflocculant is added, levels off, then rises again past a certain point as the slip becomes overdeflocculated. Potters generally aim to stop just before that curve bottoms out, a slightly underdeflocculated "controlled state of flocculation," rather than at the minimum itself, since a slip deflocculated exactly to the curve's low point is more prone to settling out or gelling unpredictably over time.