The wi5 signal is an electromagnetic radiation with frequencies in the UHF (Ultra High Frequency) band. When an electromagnetic wave encounters a barrier, like concrete from walls, a part of the incoming wave is transmitted through the barrier and other part of the wave is reflected back. The coefficient of transmission through the barrier `T` depends mainly on how permeable the barrier is to the electromagnetic field (this is called the "barrier height" - what type of material the wall is made of) and the thickness of the barrier (how wide the wall is). The reflection coefficient which expresses the ratio of the amplitude of reflected wave to the incoming wave is simply
In a way it is like when a ray of light encounters the glass from a window. A part of the light will be reflected back and another part will be transmitted. If the window is made of ordinary glass, only a small part of the light will be reflected back, but if the glass is coated with a thin metallic film almost all of the light will be reflected back. This is similar to the height of the barrier described above. Likewise, if the glass is very thick only a small part of the light will be transmitted, but if the glass is very thin almost all of the light will pass through it. This is similar with the thickness of the barrier.
Light is basically electromagnetic radiation having a very high frequency, so that the behavior of the wi5 signal passing through a wall will resemble the behavior of light when passing through the glass layer.
Simply, the wifi is able to pass through the wall because the waves are able to transmit through the walls because walls for the most part, don't affect the waves. This has to do more about waves and physics. Wifi are radio waves, and walls do block them which is why most wifi doesn't reach a huge area. Although brick and concrete walls are harder to pass through because they absorb the waves.