Line of sight means an infrared signal needs an unobstructed path between the transmitter and the receiver. Unlike radio, many IR systems do not work well through walls, closed cabinets, or other physical barriers. If something blocks the beam, the command may never arrive.
In cyber security, this matters because IR-controlled devices often rely on a simple trust model: if you can point a transmitter at the target, you can send a command. That can be a defense advantage, since physical obstruction limits remote abuse. But it can also be a weakness in shared spaces, where an attacker with brief access or a hidden emitter can trigger TVs, projectors, sensors, or other equipment without touching the network. Defenders use the same property to reduce risk by placing receivers out of direct view, adding shielding, or preferring authenticated wireless controls over exposed IR paths.