Serums are skincare products that are designed to deliver high concentrations of specific active ingredients to the skin. When layered underneath your moisturiser, serums act as a targeted treatment for wrinkles & pigmentation.
A serum is able to hydrate more effectively than even the heaviest creams. The molecules are able to truly penetrate the skin and hydrate on the deepest level, while the heavier creams more so sit on those top layers. Hydrating serums do not necessarily replace your moisturiser, but can boost the hydrating effects of your moisturiser.