Glycolic acid is a type of alpha hydroxyl acid (AHA) which is derived from sugar cane. It is commonly added to hydroquinone-containing products for the treatment of hyperpigmentation (dark patches of skin). Glycolic acid is used to exfoliate skin. This helps to remove dead skin cells and thus helps to improve the penetration of hydroquinone into the skin. It also helps to enhance the depigmentation effects. The study has shown that the combination of hydroquinone and glycolic acid is safe and effective in the treatment of melasma (a form of hyperpigmentation). 
While using glycolic acid containing products, avoid using other exfoliating agents and products containing AHA and beta hydroxyl acid (BHA) such as salicylic acid to minimize the potential of skin irritation.
Last but not least, we can never stop stressing the importance of sun protection while on treatment for hyperpigmentation. Apply sunscreen with broad-spectrum coverage (UVA and B) and reapply especially after sweating.