Hydroquinone is a cream that takes pigment out of the skin. It blocks the natural chemical process that leads to the creation of melanin, the substance that makes skin dark.
Tretinoin is a type of vitamin A that helps to increase the rate at which dead skin cells fall off and new ones appear. This makes the melasma patch fade more quickly as the pigmented cells are shed.
Azelaic acid cream appears to work by slowing down or stopping the production of pigment, the substance that makes the skin darker.