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), 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. That is why printer profiles made with paper using a lot of optical brighteners can end up printing out 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.
Before you say "Well I use a really good quality paper - I'm sure they don't use this stuff," you should know that just about every paper that has a nice, satisfying white to it has some amount of optical brightener. In fact, you usually have to hunt around a bit to find paper that specifically has no OB in it. It's usually labeled "natural white" or something similar and is noticeably less bright with a small tint of yellow to it.
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. Paper without OBA (or very little) include "silver halide" RA-4 process photographic paper, certain press proofing papers, fine art papers and other specific paper types that are marketed by paper manufacturers as having no OBA.
Before you panic and think this is the printing equivalent of hormone-injected beef, consider that this is a very reasonable way to get more white onto a paper. Without it, many of our papers would be rather drab looking by comparison, given the natural color of wood pulp and cotton. Consider that the chemicals required for bleaching a paper white might be worse in many ways than the OBA. You should be aware of some issues with this paper though.
The chemical agents in paper causing this fluorescence will 'run out of juice' over time (sort of like the pocket warmer in the bottom of my sleeping bag on our last camping trip. It was nice while it lasted.) That means that over the course of several years or even months, the apparent brightness of your paper will decrease; it won't be "glowing" with the artificial white that it does now. This is part of the reason why some people choose to print with "natural" papers.
It is interesting to note that if your print is going to hang in a museum somewhere, any ultraviolet light would most likely be filtered out of the illumination lights, so the OBA's would not work - the paper would appear its normal brightness without the ultraviolet light providing the "power" for the fluorescence.
How can you tell if your paper has OBA's?
The quickest way is to shine a black light on the paper. A black light lamp will cause the OBA's to glow in a big way. You've probably seen this effect with white clothing under a black light. Paper (or clothes) without artificial brighteners will not react to the black light at all. Black lights that screw in to normal household sockets are available, but sometimes hard to find. Your kid's "invisible ink pen" he got at the toy store probably has a black light lamp on it to illuminate the OBA's in the invisible ink.