The Follow Through is maintaining the position of the bow arm on release until the arrow hits the target. As the arrow slides along the arrow rest any movement of the bow will move the arrow.
The position of the head and body should remain steady, while the drawing hand moves backwards after the release.
It is important to not let the bow arm fall after the release, as this can become a problem when the bow arm actually starts to fall on the release, making some arrows land low on the target. Also moving the head to see where the arrow went too soon after the release can make the bow arm move sideways.