The 18% gray card reflects 18% of the light that falls on it. This 18% reflectance is the midway point (this is where the 50% comes in, not in reflectance, but in regards to the difference in stops) in regards to increments of exposure between white and black. Ten exposure zones between Zone0 and ZoneX. 5 stops from either extreme lands you on Zone V, smack dab in the middle. 18% gray.
That's the best layman's terms I can come up with.
Let's see the numbers. What reflectance are we calling zone VIII for instance?