Piggy's glasses in Lord of the Flies, I think symbolize civilization. While Piggy is not good looking and charismatic like Ralph, or powerful and ruthless like Jack, he is rational and he always brings the "civilized" point of view to the discussion.
Piggy's glasses, then, represent the clarity that civilization brings to society. Without them, Piggy can't see anything. He is blind to the truth. However, when he has them on, everything comes into focus and the "humane" choice can be made.
One of the "civilized" things the specs help with is the starting of the fire. Fire has often been linked to civilized society. When the cavemen learned to make fire, they took a step out of the savage world toward civilized life.
When Jack smacks Piggy and his glasses fly off and break, that is the parallel to civilization starting to crack. When Jack and his tribe steal the glasses, that means that the ruthless savages have stolen civilization (and therefore humanity) away from the boys.
Once the glasses are gone, it foreshadows the death of Piggy, because a civilized man cannot survive in a world that worships strength, brutality, and power.
Piggy is fat and unpopular, but he represents the intelligent and civilized aspects of the boys in Lord of the Flies. He is a major asset to Ralph, who nonetheless shuns him for much of the story. Piggy's glasses give him the look of the educated bookworm, but they also represent his physical weaknesses. The glasses serve as a symbol of power--not only intellectually, but they also have a higher purpose as the fire creator, since the boys use the lenses combined with the sun to start their fires. When Piggy loses his glasses, he loses his last semblance of power.