Wi-Fi signals experience attenuation due to distance from their source, just like any other radio. This signal attenuation subsequently causes the radio to be less effective and throughput suffers. Eventually, connectivity is lost when the signal becomes too weak. There are essentially two factors that affect signal attenuation between the access point and the client, and those are range (distance), and physical obstructions such as thick walls, glass, mirrors, and other interference from items such as microwaves, baby monitors, two way radios, sound bars and any other devices operating on the 2.4GHz frequency.
The distance or range which can be tolerated between an access point, such as the Wi-Fi Router, and any client device is affected by several factors. Some of these factors are built into the hardware of the receiving and sending devices, such as power amplifiers on transmitters to boost power, and low noise amplifiers on the receivers to make them more sensitive. An overly sensitive receiver picks up more interference, so there is a balance point where transmit power and receiver sensitivity are optimised for a specific application in the typical home.
The figure below shows a diagram of a typical single storey home, with indicators showing what strength of Wi-Fi signal can be expected.