For example you are trying yo convience someone to do some thing and finally the person agrees to do that so you ask the person again for conformation by asking "are you onboard with me"
If I am going onboard a train or ship and I ask you if you are coming gonboard with me. If you are onboard with me, you are going my way. Sometimes trying to define an idiomatic expressions is a Challenge. I guess you can say that in a nutshell, it meant "do you
share the same view/ belief/ value/ goal with me?"