To see what does preboard mean, let’s start with the much more popular term: onboarding.
(that’s true: as of 2/26/2024, the Bing search for preboarding has only about 40,000 results, while the search for onboarding has more than 2,000,000).
So,
onboarding is the stage between hire and work routine, when an employee becomes fully involved in your workflow. It starts with getting acquainted with your work routines and continues with a variety of educational courses, regular meetings, tasks, surveys, and reports, so a newcomer gradually becomes a part of your team.