From SC2Pod
Line of sight describes what your units and buildings see. Some objects on the map can obstruct line of sight from ground units, such as the tall grass in Blistering Sands or the smoke vents in Scrapyard.
In Starcraft 2, when one unit attacks another from a place where the second unit can't see the first unit, and the first unit attacks the second, the owner of the second unit will see the first unit, but can not fire on it until it is revealed by another unit seeing it normally. e.g. if a Stalker is on a ledge above a Marine and is attacking the Marine, the Terran player will see the Stalker where it is, but won't be able to shoot back until one of his units can see the Stalker.
