Basically, the one who has the most power.
If you think of a game of marbles, the intention is to win your opponent's marbles. The one who has the most marbles wins.
So Steve Jobs is the one with the most power, with an implication that he has beaten everyone else on the way to getting the most power. (Competitors in the field of computers, presumably.)
This isn't a standard idiom, by the way, just what I take from the phrase. ("He/she has lost his/her marbles" is an idiom which means "He/she is crazy" or "can't think straight.")
These are marbles, if you weren't aware: