Even though the data transmitted over Wi-Fi is digital, the actual signal uses analogue radio waves. These are susceptible to interference, much in the same way as the radio in your car. If you are driving in a multi-storey car park, or driving through a tunnel, the signal of radio will weaken. This will either cause noise or a complete dropout of the station you are listening to. The same happens to your Wi-Fi.
The image above shows the router connected to the phone line in the living room. The laptop directly above it in the bedroom has full signal strength, as generally the signal can penetrate through ceilings without any problems. The laptop in the loft conversion is much further away; this weakens the signal slightly and some performance issues may be experienced.
However, for the laptop in the kitchen, the signal has to penetrate a load bearing wall - so, despite being closer, the signal is weaker. Generally walls like this are made of more substantial material; brick, stone or concrete. This can dramatically affect the radio signals between your router and Wi-Fi device, and can cause slow or intermittent connection problems.
Other materials in your home can also cause signal degradation. Along with concrete, brick and stone, foil-backed insulation foam, some wallpapers and damp can all hinder the radio waves. It's normally quite difficult to determine which materials are installed in your home, and which are causing issues, but using a Wi-Fi repeater or relocating your router can help improve things.
Electrical devices you may have in your home could also be causing problems with your Wi-Fi network. Devices such as baby monitors, microwaves and refrigerators can interfere with the radio signal from your router; anything which communicates on the 2.4GHz frequency can interfere. Whilst it's not common for these items to cause major problems, it could be worth keeping them in mind if you are having problems resolving any issues you have.
In some situations you may find the Wi-Fi signal is better in different parts of the room. The diagram below suggests one reason why this might happen.
As in the first example, the load bearing wall is also effecting the signal in other rooms. Line-of-sight is important for radio waves, as they only travel in straight lines. As the image shows, when the signal passes through the thicker wall the signal is degraded. Moving the laptop to another part of the room, where the signal just passes through thin walls, improves the signal.
In some situations you can get 'pockets' of good and bad signal quality, so moving your Wi-Fi device around the room (if possible) can help.