Preboarding is the stage between a new hire accepting the offer and their first day, used to handle paperwork and keep them engaged before they start.
Preboarding fills the gap that used to be dead time. Instead of leaving a new hire to wait and wonder, you use these weeks to get forms signed, order equipment, set up accounts, and send a warm welcome so day one is calm instead of chaotic.
It also protects against early drop-off. People sometimes accept a role and then keep interviewing elsewhere, so staying in touch before the start date keeps them excited and reduces the chance they never show up.