Refer to figure.

A skip distance is the distance a radio wave travels. It is a distance measured along the curvature of the earth between two points where radio waves from a transmitter, refracted downwards by different layers of the ionosphere, comes back to the earth’s surface.

Radio waves from the transmitting antenna do not all get refracted by the ionosphere. Depending upon the refractive index of each of the layers in the ionosphere it is determined what happens to the transmitted radio waves. Some of them are absorbed, some are refracted and some manage to escape to the next layer.
At this higher layer usually the radio waves bent down to earth again.

Because of the different heights in the ionosphere the radio waves hit the earth’s surface at different points, that’s why there is referred to a skip distance. Skip distance is greatest during the night when the ionosphere is the highest.