Line of sight
Line of sight (sometimes written line-of-sight or abbreviated to LoS) is an invisible straight line used to determine whether entities can see other entities in the world. (If the line of sight is unobstructed from a viewer to a target, it is "established", and the game determines that the viewer can see the target.)
LOS is blocked by all Solid geometry (Models, func_detail brushes, etc.) unless the brush/model is has a SolidMask that is NotSolid to LOS, ie Transparent.
The Block LOS tool texture is specifically designed to block NPC LOS.
NPCs can trigger Outputs if it finds or loses line of sight to an enemy (OnFoundEnemy/OnLostEnemyLOS) or specifically the player (OnFoundPlayer/OnLostPlayerLOS).