Line of sight, abbreviated as LoS, refers to the action of preventing an attack of a player by making the targeted player to locate itself behind an object. As the targeting system recognises the presence of an obstacle, the player attacking will not be able to use the skill on the target due to the object obstructing the way.
This is mostly used in open world and Arenas, when a class with long-range attacks casts a hard-hitting skill and tge target wishes to interrupt the effect. This is only applicable to players, as mobs's attacks will always go through any obstacles.