A "brightener" is something that makes something else brighter or more vibrant. It is like a special product or ingredient that brings out the brilliance or luminosity of an object or surface. Full definition
It is also called ALL free & clear but it has a red label that marks it as «free of optical brighteners». (thirstiesbaby.com)
But also because of its natural base and the fact that it is free of brighteners, dyes, and synthetic fragrances, it is also very gentle. (followmum.com)
What you may not know is that avocados also contain Vitamin C, a natural skin brightener for any sunspots you may have recently acquired (ahem). (theeverygirl.com)