In short: everything.
For one, vitamin C promotes collagen production, which has the potential to thicken the dermis, diminish fine lines, and is essential for firm, youthful skin.
On top of that, vitamin C is an antioxidant, meaning it protects skin cells from damaging free radicals caused by UV exposure.
It also inhibits melanin production in the skin, which helps to lighten hyperpigmentation and brown spots, even out skin tone, and enhance skin radiance.
Finally, vitamin C helps to repair damage from sun exposure and collagen loss by encouraging healthy cell turnover and regeneration.
The antioxidant properties of vitamin C (ascorbic acid) and its role in collagen synthesis make vitamin C a vital molecule for skin health.
Dietary and topical ascorbic acid have beneficial effects on skin cells, and some studies have shown that vitamin C may help prevent and treat ultraviolet (UV)-induced photo damage.
Traditionally, topical vitamin C products contain L-ascorbic acid, the “grandparent” form of the ingredient, but it is also famously unstable.