Image: Naughty Dog / PlayStationOfficial screenshot from Naughty Dog / PlayStation.
Lights Out is the bronze trophy for switching off the hunters' searchlight generator in Pittsburgh without being spotted. It happens at night at the guard post by the bridge, when Joel and Henry sneak toward the gate while Ellie and Sam wait behind.
After you quietly deal with the first two hunters, keep out of the searchlight beam and work your way to the generator that powers it. Switch it off while you are still undetected; the hunters then go looking for the fault.