Glycolic acid is a type of alpha-hydroxy acid (AHA). It's a chemical exfoliant used to exfoliate dead skin cells by breaking the glue bonds between skin cells. This allows the dead cells to slough off easily and leaves your skin smooth and clean. It also helps stimulate collagen production.  Hyaluronic acid is a humectant, which means it draws moisture to the skin. Both glycolic acid and hyaluronic acid (HA) should be used to improve aging skin. You should use the glycolic acid first (following directions on the bottle) and then once it's rinsed off, and before you apply your moisturizer, apply a light layer of HA to hydrate the skin.