Not an acid in the classic, or chemical sense! Hyaluronic Acid is a naturally occurring part of the human body (found in greatest concentration in the vitreal humour and synovial fluids.) It is a humectant, having a very high capacity for holding water. Because of that, it is used in serums, drops, and supplement to give structure and moisture to tissues.
Hyaluronic acid is used by some people to keep collagen synthesis up, retain skin moisture, improve elasticity, and aid in healing free radical damage. Higher levels of both of these compounds are related to supple skin resistant to developing find lines and wrinkles.