WiFi signals aren't straight beams. It's better to compare it to ripples in a pond when you throw a pebble in. The ripples propagate in all directions from the source. They get weaker the further they get. Obstacles along the way can block the ripple or alter it and the ripples may get reflected by barriers.
WiFi signals work in much the same way, they're emitted from the router or your device and spread out in many directions (note: the antenna in a router typically focuses the direction of the spread to be closer to a flat plane than a sphere and there are special directional antennas that turn it into a more beam-like shape). If your device and the router are in direct line of sight with each other, than part of the signal will travel that straight line path.
But at the same time, the signal can cross through solid matter, although it loses strength far more rapidly than when it crosses through air (depending on the material) and the signal can also reflect off objects in the room, including walls, floor and ceiling. Different materials provide a different level of reflection. Especially metals are very good for reflecting WiFi signals.
That means that when your device is connected to a router via WiFi, it will receive the same signal from a number of directions, with different strengths and delays. Just like ripples in a pond, WiFi signals coming from different directions can interfere with each other, causing the signal to be either amplified or reduced. This is called multipath interference and the effect of it is very location dependent. In areas with a large amount of multipath interference (for example due to the presence of metal objects and surfaces), moving the WiFi device by relatively short distances can cause a noticeable change in the strength of the WiFi signal.
Wifi travels like a wave. Actually it's an electromagnetic wave, just like radio, X-ray, and visible light. It is subject to reflection, refraction, and diffusion just like those waves.
So, when you send out data by wifi, it goes out as an expanding (roughly) sphere (based on the shape of the antenna).