Locke told us what this was all about in the second episode by Brad

My wife wanted to watch Lost from the very beginning again as she has only been through it once. It is the third time for me. When we got to Season 1, Episode 2, I believe Locke told us what this is all about right there on the beach playing backgammon. When Walt asked him about the game he was playing and asked him if it was checkers, Locke proceeded to tell him about the backgammon game. He said it was older than Jesus Christ, and he said "two players, two sides, one is light, one is dark." My theory is that this seemingly insignificant line at the time, now takes on a HUGE meaning. Perhaps this "game" between Jacob and MIB is older than time itself. The two sides, one is light and one is dark, line certainly takes on a very significant meaning now. Could it be that this whole thing is like a game of backgammon. Go to Wikipedia and read about it. There's even something in backgammon called the "Jacoby! rule." Hmmmm.. It states: "The Jacoby rule allows gammons and backgammons to count for their respective double and triple values only if the cube has already been offered and accepted. This encourages a player with a large lead to double, possibly ending the game, rather than to play it to conclusion hoping for a gammon or backgammon. The Jacoby rule is widely used in money play but is not used in match play."

Notice the player with a large lead can DOUBLE (as in double Locke)? I may be way off but I think this is a big key.