Glycolic acid is a chemical exfoliant. The acid breaks down the 'glue' holding dead skin cells together and promotes the creation of new cells. It boosts production of hyaluronic acid and collagen for improved hydration and elasticity. All that leaves you with a softer, smoother, and more nourished complexion.
Glycolic acid is an alpha hydroxy acid (AHA) that comes from sugarcane plants. As far as acids go, it's a pretty gentle choice. The molecular structure means this acid penetrates the skin more than most AHAs. That makes it more potent, working right down to the skin's deeper layers.
Most people tolerate glycolic acid well since it's quite a gentle acid. It's available in many forms, from small amounts in over-the-counter creams right up to medical grade products.