Refer to figure.
The speed of a surface wave is affected by the surface over which it travels, going faster over water than over land. This change of speed, means that the wave is refracted at low altitude as it passes over a coastline.

Plotting your position from a series of bearings taken from a coastal NDB, will place the aircraft nearer to the coast than it actually is, because the effect of coastal refraction creates a false impression of the bearing of the NDB from the aircraft and when the reciprocal is plotted on the chart, the aircraft plotted position is closer to the coastline than it realy is.

This effect is worse the further back from the coast the beacon is sited. It can be minimised by choosing beacons closer to the coast or flying at a higher altitude. It can be avoided by taking bearings at right angles to the coast, when no refraction will occur.

The effect described above is called "Shoreline Effect" and is greatest with radio wave propagation at an acute angle to the coast with the aircraft flying at low altitudes (such as below 6000 ft).