Hydroquinone is simply a "bleaching agent" or a "lightening agent" for skin concerns such as hyperpigmentation, like melasma (pregnancy mask), post scarring from surgery, acne scars, basic injuries, sun damage, etc.
This ingredient does travel through your bloodstream so it is advised to use the product no more than 3 months!!! Not while you're pregnant either.
I suggest that the lightening product you apply on your face works deeper within the skin in the layer that produces melanin. There are damaged cells that over produce melanin...they keep replicating like they should but the are damaged! Bad cells make more bad cells. So we need to fix the bad replicating cells into healthy replicating cells...overtime the layers of our skin grow and shed (image) resulting in healthier skin from within! That is also why exfoliating is an added must.
Info and a picture to show you an example: http://www.theskinencyclopedia.com/index.html