In the literal sense, you are on board a ship or an airplane. The ship leaves at 2 p.m. Everyone must be on board by 1:30.
In a figurative sense, you have accepted a situation, a plan, a program. I think that Jim in on board with the plan we developed this morning to fix the server situation.
I used the expression just today while writing to a friend who had been out of touch with all of his friends for a couple of years and has recently returned. I said, "Glad to have you back on board".