When a direct line of sight exists between the transmitter and the receiver antenna, we say the receiver receives a direct signal. This signal is usually the most powerful signal the receiver can receive from the transmitter. We say that there is a Line of Sight (LOS).
The receiver also receives reflected signals when the transmitted wave is reflected on obstacles. We call these signals indirect signals. The receiver adds all the indirect signals to the direct signal in the receiving antenna. The received signal is the sum of all the received signals. When the sum of these signals results in a distorted signal where the content cannot be demodulated, we have a problem. The problem we call phase distortion.