Onboard means to bring someone new into an organization or project and help them learn what they need to know. When a company onboards a new employee, they teach them the company's systems, introduce them to coworkers, and explain their responsibilities. When a teacher onboards new students at the start of the school year, she shows them where things are, explains classroom routines, and helps them feel comfortable.

Good onboarding makes a big difference. Imagine joining a soccer team mid-season: if someone takes time to explain the plays, introduce you to teammates, and show you how practices work, you'll feel confident much faster than if everyone just expects you to figure it out yourself.

The word also works as an adjective describing something that's part of a vehicle or vessel. An airplane's onboard computers control flight systems. A ship's onboard generator produces electricity.

In casual conversation, when someone says they're onboard with an idea, they mean they understand it and agree to support it. If your friend proposes building a treehouse and asks, “Are you onboard?” she's asking whether you're willing to join the project.