Wikipedia[1] has nothing to say about it ... in fact, the wiki definition of "fuchsia and magenta are exactly the same color, made by mixing blue and red light at full and equal intensity" makes it seem not that interesting at all ...
By a color "not being in the rainbow" we mean that if you take light and spatially separate its different frequencies (like what happens in a prism or droplet of water), none of those individual frequencies will produce that given color response. And no, it's not that interesting. Almost all color perception experiences involve stimulating a patch of retina with multiple frequencies; we've given names to many of those experiences.
Take yellow. You could experience yellow by seeing pure yellow light which is at the yellow wavelength. The other way to experience yellow is by seeing a mixture of red and green wavelength light.
Your eye can't tell the difference between these two types of yellow. The yellow as reflected off a banana is "real" yellow. The yellow you see coming out of your computer monitor is actually just a mix of red and green.
Magenta is interesting because the only way to experience it is by a combinations blue and red. There is no "real" magenta.
This video explains this concept very well: