According to one of the developer conference videos "wide color" is how apple deals with a display that has more gamut than sRGB. For all displays apples default is sRGB but some displays can display more...so they developed a scheme that allows one to specify color reference numbers greater than 1 and less than 0.
As an example one could specify pure red as (1,0,0) and that will work fine. On the newer systems you can specify a larger number than 1 and number less than zero say...(1.2, -.25,-.25). I did not watch the lecture all they way through to see what the limits are or how the developer would determine what the limits are for a given setup.
I hope that helps a bit...if I get more time perhaps I'll go through the lessons from wwdc further...
--Jeff
I should note that the presentation seems more iOS focused so there may be a slight difference for macOS.