Traditional wifi uses omni-directional antennas that propagate in spheres (roughly). If you want to see a good example, turn on a few light bulbs, and notice how they all overlap in coverage. Fifteen light bulbs will have a *lot* of overlap, and you'll start stepping on each other. 2.4GHz wifi is particularly bad this way, due to neighboring channels overlapping.
Directional antennas propagate in cones (roughly, again). A good example of this is a flashlight. You can turn on a whole lot of flashlights before you start seeing the same amount of overlapping. The downside is that it's hard to light up a room with a flashlight, so you have to start doing a lot of tricks to move a portable device around but maintain a connection.
I can't really explain RF propagation with a car analogy, so you'll have to settle for flashlights and light bulbs.