It meant there are two things in the world: darkness and light.
It also means that the two represent many things, like good vs. evil; moral vs. immoral; trust vs. distrust; caring vs. apathy; friendship vs. enemy.
When you look at Locke's face above, you notice that he is scarred above and below his right eye. In Egyptian mythology, the Eye of Horus was a scar on the left eye of a sky god. In one story, Set and Horus fought over the throne after Osiris' death. Set gouged out Horus' left eye, which was restored by the magic of Thoth, the moon god. So the dark piece could be the moon and the light piece the sun, Ra, the most powerful god who had to traverse the dangerous underworld each night.
For the left eye of Horus represented "protection, royal power and good health."
But since it was a mirror image on Locke's face, would it represent the opposite?
"Insecurity, servitude and poor health."
In many ways, insecurity, servitude and poor health was Locke. He was clearly insecure in his own self, his skill set, his dreams and his handling his own expectations. He thought of himself as a great man, but never had the drive to be his own boss or leader. He was boxed in various meaningless tasks like a clerk at a box company, or a pest control specialist. And his poor health, his disability, was direct result of mistrusting his con man father and his inability to defend himself.
In many ways, Locke was set up from the very beginning to be the series Fall Guy. He would never be the true leader. He would never have any true power over other people. And he would not live a happy or healthy life. He embodied the dark fears of most people's subconscious. Even when things are going well, there is a hint of pending doom, despair or questioning in the back of one's mind. "This has to be too good to be true." Self-doubt at times turns into real doubt on the road to failure.
It is ironic that Locke was trying to teach Walt, a young black child who had no mother and a stranger as a father, the symbolism of backgammon. Walt would wind up with a better fate than most island castaways: he would find some security with his grandmother, he would have a chance to control his own future, and he was in good health the last time Locke saw him. The time Locke could have tried to convince Walt to give up his new life and return to the island with him. But Locke did not even try to convince Walt to return to the island. It really was the clearest black and white decision Locke ever made in the show.
