Boresight is a direction, a line, from the antenna where the signal (reception) is strongest (That is the optical axis of the antenna. Further you get from the boresight, weaker the signal will be).

No.
Boresight is the line, where the system is guiding the missiles.


Both screens are for elevation (Epsilon).
Left is normal mode, right is H<5.

On the right side, with H<5, the boresight of the system (where it aims) is moved 3 degree down from the center of the antenna beam.
Now, you can aim at -3degree while the system is locked down at 0 degree in elevation, and also the clutter is less in the Azimuth screen.

Am I confusing enough?