means The process of integrating a new employee, customer, or user into a company or system, typically through training, paperwork, and account setup.

from A 20th-century business coinage built straight from the nautical and aviation phrase 'on board' — the idea of bringing someone aboard a ship or plane. By the late 1900s the metaphor had drifted from gangplanks to HR departments, with 'onboard' verbed into 'onboarding' to describe hauling fresh recruits into the corporate vessel. The maritime spirit lingers: you're still being welcomed aboard, just with a laptop instead of a lifejacket.