LoS is an acronym that stands for Line of Sight. Most spells and ranged abilities must have line of sight to attack a target. This means that if a mob is standing around a corner, you cannot shoot it because you don't have line of sight on it.
Understanding and making the most of LoS is extremely useful in both PvE and PvP encounters. For an example of a use of LoS in PvE, see Pull.