Well, that's schlick's approximation, so it's safe to assume that it says "reflectance at normal angle". In essence that's your specular value.
It's not a bad idea to check out how the actual fresnel calculations
work. the value of R (in the context of computer graphics it's more generally referred to as f0) is calculated from the index of refractions of the material and the medium it's in. Video games all take place in a vacuum so the equation gets simplified to
as an example here's the reflectance at normal incidence of window glass (IOR 1.520)
( (1-1.52)/(1+1.52) )^2 = 0.04...
transform that into an RGB value..
0.04*255 = ~10
piece of cake. it's a bit more involved for non-dielectric materials but i hope this explains what the value means.