Line of sight refers to a character's ability to hit a foe using an arrow or a projectile. In order for rangers to hit with their bows, paragons to hit with their spears, or spell-casters to hit with their staves and wands, they must have line of sight towards their target, i.e. there must be no physical environmental obstacle in the way. This also applies to certain spells that send out projectiles such as Lightning Orb and Stone Daggers and offensive spirit attacks
Understanding how line of sight works and affects hitting or missing is an important element of success when playing the game. Often, great amounts of damage to a character can be avoided by just going around a corner. As a general rule, if your character has line of sight to another character, that character has line of sight to your character. Line of sight is not obstructed by other characters, only actual immobile physical barriers that a character cannot walk through.
If a character has line of sight, their projectile/arrow will reach the target, whereby it may still miss as a result of being blocked or other factors. On the other hand, if a character does not have line of sight, the game will play a clicking sound and a warning message that view of the target is obstructed. In addition the word "Obstructed" will hover over the target instead of the damage inflicted.