What does Optical Brighteners (also Optical Whiteners) mean?

Chemicals that make fabrics appear to reflect more light and make them brighter.

Heddels explains Optical Brighteners (also Optical Whiteners)

As their name implies, optical brighteners, also known as fluorescent whitening agents, are dyes that increase the whiteness and brightness of fabrics, plastics, inks, and detergents.

They do so by absorbing (invisible) ultraviolet light and re-emitting it as visible light. Optical brighteners are helpful in masking yellowish hues that may appear in fabrics during either the manufacturing process or regular wear.

Additional Resources

Optical brightener whitening agents: