Line of sight, commonly abbreviated LOS, is an unobstructed line drawn from your character to an enemy. Most abilities or spells and ranged attacks require that you have line of sight to your target to cast them, and will give a message when you don't, canceling the activation.
When casting a channeled spell on an aggroed enemy, when that enemy moves in such a way as to block your LOS at any point in that cast, it will cancel your spell. For example, if you are on the ground, and you begin casting Smite or any such spell on a melee mob that is in an elevated shack, when that mob comes at you, he will cross behind pillars, most likely, and at that point your cast will be canceled. (For those that recognize the region, the islands in the southeast of Durotar have such shacks.) For channeled spells, however, the mob may not be attacked one second or another, or perhaps the effect that the spell gives (such as Mind Flay) may not proc, but the next tick where there is no obstruction during that span, it will continue.
Obstructed LOS is often inconvenient, forcing the player to move to regain LOS before attacking an enemy or healing an ally. One can also intentionally obstruct LOS, e.g. by running around a corner, to force an enemy to move to a more advantageous location.
LoS can also be exploited by players to pull difficult groups. The puller (usually the tank) should attack one mob, then quickly run behind an obstacle. This breaks the monster's LoS to their aggro target, causing them to follow the puller around the corner. It is imperative that no other characters draw aggro while the mobs are running - DPS should not attack and healers should not heal. As soon as they turn, they can be tanked at short range. This is very useful for tanking casters, who otherwise stand still and attack party members at range. "To LoS" is to employ this strategy.
- Uneven terrain, staircases, walls, and other environmental details can obstruct LOS, causing ranged actions to fail.
- Line of sight appears to be drawn from midsection to midsection, so short characters get the "No Line of Sight" message slightly more often.
- Terrain (that is, dirt and trees) only block Line of Sight indoors. All dirt and most trees do NOT block spells or abilities when fighting outdoors.
- Latency and/or packet loss often negatively affect line of sight. This will create an effect in the game where you "think" you can see your target, but because your game is lagging, your enemy will already be hiding behind an obstruction, or warping to a different part of the map.