A reflector adds no light of its own; it redirects light that would otherwise be lost. Its strength depends on its distance from the product and on its surface. White gives soft, neutral fill, silver gives brighter and harder fill, and gold warms whatever it lights. A black card does the opposite: it absorbs light on the shadow side and deepens it, which gives shape to a pale product. For product work a sheet of white foam board or card does most of the job.
With a window or a softbox on one side, the reflector stands on the other, close to the product and angled towards it. On a model it is held low in front to lift the shadows under the chin and beneath a hat brim. On a dark garment it brings back detail in the folds, which would otherwise turn black. A small folded card behind a bottle or a glass puts a bright edge along its outline and separates it from the background.
Too much fill flattens the light, and the product starts to look pasted onto its background. The reflector's colour also travels with its light: a gold surface turns a white shirt cream, and a coloured wall near the set acts as an unplanned reflector that tints a white product. A reflector softens a shadow but does not remove it; where a rule allows no shadow at all, the rest is removed in the cut-out.