The setting of Lord of the Flies, by William Golding, is a tropical island, which means it is covered with all manner of lush, green, growing foliage and trees. In chapter three of the novel, Jack is frustrated because he has still not been able to catch a pig, though he hunts constantly.
Part of Jack's strategy is to follow the trails left by the pigs. He follows them through grass and dirt and foliage, hoping he can sneak up on one and kill it. In this passage, Jack is on the hunt and searching the ground diligently for signs of the pigs.
By the trunk of a vast tree that grew pale ﬂowers on its grey bark he checked, closed his eyes, and once more drew in the warm air; and this time his breath came short, there was even a passing pallor in his face, and then the surge of blood again. He passed like a shadow under the darkness of the tree and crouched, looking down at the trodden ground at his feet.
"Trodden" means stepped or trampled on, as in ground that has visibly been walked on by many or by something heavy. (Anyplace the boys on the island regularly walk would soon become trodden and make a path.) The next sentence talks about Jack finding pig droppings, so we know that, in this case, the ground was trodden on by some pigs who routinely travel over this ground.