Wi-Fi signals emitted from the router are normally carried by microwaves to your laptop or smartphones and therefore, the devices are able to connect to internet. Most of the modern communication devices nowadays are carried by electromagnetic waves.
Electromagnetic waves are used because it can travel through walls therefore you can make or receive phone calls from inside the building. On top of that it reflects from the earth’s upper atmosphere. This is because at high altitudes the atmosphere becomes a plasma. It is a state of matter whereby atoms are split apart and electrons no longer bound to their parent nuclei. Moreover, they react pretty well with electromagnetic fields. If it is set to a low frequency, it will bounce the radio signals around the world and thus extending its range.
The electromagnetic emitted from the antenna of your wireless router is caused by small current oscillating at 2.4GHz. Therefore, when you develop a model by simulating a current like that and allow it to oscillate, you might be able to map the wi-fi signal strength as you move the router. Sometimes, you might find the waves have stopped changing and flicker in the same location. This is what we called standing wave and normally it will cancel one another.
The dark spot on the map showing low wi-fi signal. If the reception is poor in a location and there is a slight change in the router’s position, it might produce significant movement in the signal strength and the dark spot will also move.
The below video will explain further.