The propagation constant is the rate at which the amplitude of a wave diminishes as it propagates outward from its source. The practical effect of this diminution is that wave amplitude drops at a predictable rate as the wave moves along a line of transmission.
The metric used to measure this observed decrease in amplitude it the neper. A signal is said to lose strength at a rate of nepers per meter. In each case, the propagation constant per unit of linear travel can be expressed as the natural logarithmic of ratio of the sending end current to the receiver station.