You are referring to the end of Chapter 3. Simon, after helping Ralph build huts, goes away and walks towards the forest. Walking through "acres of fruit trees", the group of littluns who have run after him catch up with him, and get him to pick the fruit that they can't reach:
Here the littluns who had run after him caught up with him. They talked, cried out unintelligibly, lugged him toward the trees. Then, amid the roar of bees in the afternoon sunlight, Simon found for them the fruit they could not reach, pulled off the choicest from up in the foliage, passed them back down to the endless, outstretched hands. When he had satisfied them he paused and looked round. The littluns watched him inscrutably over double handfuls of ripe fruit.
It is interesting that the littluns are described in a way that diminishes their humanity: they are not able to communicate with Simon clearly and are very greedy in their desire for fruit, glutting themselves as much as the can. Notice too, what this shows about Simon. He is clearly very different from the majority of the boys. He is generous and kind, and while most of the older boys would taunt and be cruel to the littluns, he is prepared to help them.
In Chapter Three of Lord of the Flies, the littluns discover acres of trees in bloom with both flowers and ripe fruit waiting to be picked. They are eager to consume the fruit but are too small to reach the branches, and so they beg Simon to help them. Simon agrees to do so and plucks the "choicest fruit" for them so that all of the littleuns have "double handfuls" of the fruit. Simon then walks off on his own to survey the land and the abundance that this strange island seems to provide to the stranded boys.
This seems to suggest that Simon has an innate goodness that the other boys simply do not possess. He appreciates both the earth and humanity and is willing to help out when others would take advantage of the situation. Simon also seems to act as a steward of the natural world.