A skip distance is a distance on the Earth's surface between the two points where radio waves from a transmitter, refracted downwards by different layers of the ionosphere, fall.
At this higher layer, there is a possibility of this radio wave being bent downwards to earth again.
This bending happens because each layer of the ionosphere has a refractive index that varies from that of the others.
[2] Because of the differing heights of refraction, or apparent reflection, the radio waves hit the earth surface at different points hence generating the skip distance.
Skip distance is greatest during the night when the ionosphere is the highest.