Optical brighteners are additives that paper manufacturers put into paper in order to help a paper look "whiter." They are also called optical brightening agents (OBA), fluorescent whitening agents (FWA) or sometimes "artificial whiteners."
In order to make paper appear brighter, it is common for most paper manufacturers to add certain chemicals to the paper which can take invisible ultraviolet light and cause it to re-emit in the blue spectrum - or fluoresce - at a point that is just barely within our ability to see. While our eyes see this as a brighter, blue-ish white - a light measuring instrument will only see this as a different form of blue. For this reason, printer profiles made with paper using a lot of optical brighteners can produce images that have a yellow tint to them. The profile is trying to correct for what it sees as too much blue in the paper.
Virtually every paper that has a noticeable bright white appearance has some amount of optical brightener. Examples of paper with OBA in it are common office bond paper, Epson Premium Matte, Luster, Glossy, etc., and most every other brand of commercial inkjet paper. The chemical agents in paper causing this fluorescence will lose their effectiveness over time so that over the course of several years, the apparent brightness of the paper will decrease. It won't be "glowing" with the artificial white that it did when new. This is part of the reason why some people choose to print with "natural" papers.