Glycolic acid is part of the group of Alpha Hydroxy Acids (also known as AHAs) and is often used in many chemical peels among other skincare products. Its small molecular size makes it capable of dissolving sebum and other particles holding skin cells together. This causes old cells to be removed at a much faster rate; bringing in newer skin cells to replace them. It is effective in helping to clear up acne since it removes sebum (a compound that our skin secretes and which many acne sufferers have an excessive amount of) and also helps to unclog pores. The faster cell regrowth means that one may see a reduction in fine lines and acne scars as well.
Glycolic Acid may be a harsh chemical to use for some people and may lead to irritation of the skin. If this is the case for you, you might want to check how much glycolic acid is contained in your skincare product, replacing it with one with a lower percentage of the compound, or just avoiding it all together if you find that you have really sensitive skin. Be sure to avoid using around the eye area, as skin is much more delicate there. In addition, it is best to wear sunscreen especially when using products with this acid, as it is known to increase skin sensitivity to sunlight.