While Boone is a companion, hostile targets are highlighted whenever the player is actively aiming.”
This perk will highlight all hostile enemies in red while you are "actively aiming". This means using iron-sights or a scope. Enemies will glow green instead of red when using a night-vision scope. This makes enemies easier to spot and attack, especially at long distances or at night. This perk does not, however, highlight cloaked enemies, such as nightkin.