Refer to figure.
Radio waves can propagate in many different ways. The most common is as a "space wave". This is what most people think of when it comes to radio transmission, as this is line of sight, meaning that the radio waves travel in straight lines only.

Another propagation path is via ground waves, which "cling" to the surface and follow the contours of the Earth.

The third method of propagation is via sky waves, which are space waves that interact with the charged particles in the ionosphere. When these waves reach the ionosphere, the subsequent ionospheric attenuation refracts the waves, changing their course, until the waves are heading downwards again. This is often referred to as "reflection" by EASA, so do not be confused; it is refraction that occurs, it just looks like the reflection from afar, and they use the terms almost interchangeably.

A problem can occur called the night effect, which is most prominent during sunrise and sunset, where the changing sunlight modifies the ionisation of the ionosphere, and causes the sky waves to "bounce off" the ionosphere at sharper angles, greatly increasing the skip distance. This means that some radio waves that can travel as both ground waves and sky waves (such as the MF frequencies used in NDBs) can begin to interfere with each other.

This interference is called "fading", and is because the two signals may; be out of phase from each other, have different polarities, or be arriving from different directions (particularly problematic for NDBs).