Could anyone explain the two-ray ground-reflection model in plain English
Whenever there are multiple paths from transmitter to receiver, each of these paths combine at the receiver. When the paths are of different length, then the phase of each path at the receiver differs in phase. If the phase of the two paths are identical, then they add together and no fading occurs. When the phase of the two paths differ by 180 degrees, then they cancel each other and fading occurs.

In general, this is called multipath interference. The two ray ground reflection model is a mathematical formulation of one kind of multipath interference when the interference is considered to consist of two paths:


from transmitter to receiver directly
from transmitter, reflected off the ground, to receiver