The bringing of a newcomer to the point where they can act. Onboarding is where a system says what it expects of people.
Old English on (upon) + bord (the side of a ship, a plank); the old image of bringing a person aboard a vessel, extended in 20th-century business English to any bringing of a newcomer up to speed.
The stretch between arriving and being able to act. What the house asks of a newcomer is set down here, plainly, before anything else is asked of them.