Line of sight
Line of sight (or LoS) refers to the player's visibility of their surroundings. Many skills in Guild Wars 2 can only be used successfully if its targeted area, enemy, or ally is within a player's line of sight. For example, all projectile skills require line of sight and become Obstructed if there is terrain between the missile and its target. If a skill is obstructed it will have no effect on its target, similar to a skill that missed. On the other hand, skills such as Wave of Wrath do not require line of sight and can be used through gates, walls, pillars, etc.
While in combat, hostile NPCs will always attempt to maintain line of sight with enemies. Players can use this mechanic to their advantage by forcing ranged foes, who are normally difficult to position without knockbacks or pulls, into melee range.