A humectant that can attract and hold an incredible amount of moisture, you’ll frequently see it in the form of “sodium hyaluronate” in ingredient lists. Sodium hyaluronate actually has greater chemical stability than hyaluronic acid (HA) and won’t pull moisture from the skin in case you live in a drier climate (source).
Since HA is meant to attract and hold moisture, it typically sits on the top layer of your skin rather than get absorbed into the deeper layers, but there are now lower weight HA molecules such as Hydrolyzed Hyaluronic Acid that can be absorbed. Studies have found that the use of sodium hyaluronate in combination with lower weight HAs can help improve the signs of aging (source).
HA also occurs naturally in our skin but the amount decreases with age, resulting in drier and less plump skin as we age. Sun damage can also result in the decrease of HA concentration (source).