The 18% gray card reflects 18% of the light that falls on it. This 18% reflectance is the midway point in regards to increments of exposure between white nd black. The 50% refers to a difference in stops, not the actual amount of reflectance. Ten exposure zones between Zone0 and ZoneX. 5 stops from either extreme lands you on Zone V, smack dab in the middle, 50% percent of that range from either end. 18% gray.
That's the best layman's terms I can come up with.