There's a pretty simple explanation and a lot more complicated explanation to this.
The simple explanation is this: at very high frequencies, radio waves behave more and more like light, so the radio stations have to be able to "see" each other. For instance, if you are using a laptop with your Hub, you may be able to use it perfectly on one place where the laptop has a direct line through clear air to the Hub, but the signal may disappear if you have a wall or even a tree between the laptop and the Hub.
The more complex and accurate explanation has to do with the "Fresnel zone" between the two radios. The Fresnel zone is a football-shaped area of space between the two radios (e.g. the Hub and your laptop, or the Hub and another Hub, or a Hub and the Receiver). The size of the Fresnel zone (the diameter of the middle of the football) is dependent on the frequency of the signal and the distance between the radios. For instance, at 2.4 GHz (WiFi's frequency) and 2.5 miles, the radius of the Fresnel zone is approximately 36 feet, meaning that, to have a fully unencumbered Fresnel Zone, a line drawn between the two radios would have to be 36 feet away from any obstruction at the midpoint (1.25 miles from either end):
Now, it's important to understand that you don't have to have a completely clear Fresnel zone to have a good connection; in fact, about 60% is good enough. This means that the notion of "clear line-of-sight" actually is a bit muddy. You can have a tree between the two radios so that you cannot see one radio from the other, but still have a good connection because the tree is only blocking 60% of the Fresnel zone. On the other hand, you may have a line of trees or a building that is exactly the same height as the antennas of the two radios so you CAN see one antenna from the other, but you may not be able to create a reliable link because the Fresnel zone is only 50% open.