Yes, it is ordinary table salt, and no, it is not filler. Salt has one very specific job in a shampoo: it controls how thick the product is.
The cleansing agents in a shampoo form small spherical clusters when dissolved in water. Add salt, and those clusters change shape from spheres into long, thread-like structures that entangle with one another. That is what makes a shampoo thick rather than running like water. The adjustment is precise: too little salt and the product is thin, too much and it suddenly turns thin again.
The texture is not only cosmetic. A shampoo that stays in your hand instead of running off is one you use less of, and it distributes better through the hair.
