What is the meaning of the term “picket fencing”?

When you are transmitting or receiving while in motion (such as driving a car), the radio waves take multiple paths bouncing off buildings, hills, and other vehicles before reaching the antenna. Because these paths are of different lengths, the signals arrive at slightly different times and randomly cancel each other out or reinforce each other. As the vehicle moves, this phase cancellation happens rapidly, creating a fast fluttering or chopping sound on the audio. This audio effect sounds very much like the noise made by dragging a stick along a wooden picket fence, which is how it earned the nickname 'picket fencing.'