Non-line-of-sight propagation is when a signal does not travel in a straight line from the sender to the receiver.
Imagine you are playing a game of telephone with your friends. You whisper a message to the person next to you, and they pass it on to the person next to them, and so on until the message reaches the last person in the line. If every person in the line can hear the person next to them clearly, the message will be transmitted perfectly through each person.
However, if there is something blocking the path between two people, like a wall or a tree, the message might not make it to the next person clearly. They might only hear part of the message, or they might not hear it at all. This is what happens in non-line-of-sight propagation.
In the world of technology, signals can be transmitted through a variety of materials, like air or walls, but those materials can cause the signal to bounce or scatter, which leads to a weaker signal or no signal at all. This is why you might have trouble getting Wi-Fi in certain parts of your house – the signal has to pass through walls and other obstacles, which can weaken it.
Scientists and engineers are always working on ways to improve non-line-of-sight propagation to ensure that signals can travel as far and as clearly as possible. They use things like antennas and amplifiers to boost the signals and reduce interference so that your smartphone, computer, or other devices can work as intended even when there are obstacles in the way.