Line of Sight simply means a straight line along which an observer has unobstructed vision of an object. This principle applies greatly when you set up an outdoor Yagi antenna. You will have to make sure the Yagi antenna has a clear or straight line of sight along which you have an unobstructed vision of the nearest TV transmitter.
Therefore, a clear line of sight enables you to receive the TV signals clearly through the outdoor Yagi antenna. However, if you have a tree or hill acting as the obstructed object, chances are you will least likely be able to receive signal or no signal at all which in most cases causes scrambled channels on your Playbox.
The diagram below illustrates the explanations above: