"I want you to be on board with what I'm gonna say" = "I want you to agree with what I say" I would say it's a phrase...and a metaphor! lol! can never really tell the difference! "Be on board with" means to "be in agreement with" or "be willing to do (sm)" for e.g. "I assume you're on board with my new ideas for the future of our company" "of course, I think they are great" Similar to "count (sb) in" e.g. "anyone wanna go to the cinema this weekend?" "you can count me in" OR "I'm on board"

It means to agree with you.

Hi, I'm the person who asked this question. So is this just a metaphor? rather than an phrase or idiom?