The expression "get on board" can mean two things, depending on context:
To physically enter a vehicle, ship, plane, or train.
Example: "Please get on board quickly, the train is about to leave."
To agree with an idea, plan, or project and start supporting or participating in it.
Example: "We’re launching a new marketing campaign—are you ready to get on board?"
In the figurative sense, it’s like saying "join us and support this plan" or "accept the proposal and take part in it."