A talisman is believed to be something that protects the person holding it from evil spirits. In Lord of The Flies, although Piggy does not really believe in the existence of the Beast, he is terrified of Jack and his tribe and he is in awe of the conch - the symbol of everything he has tried to hold on to and which has guided him, to the best of his ability, to do what "grown-ups," and certainly his auntie, would do. Unfortunately, moments after holding out the "talisman,"-the conch (ch 11)- Piggy is dead and the conch is in pieces so, it did not protect him.
After Piggy's death, Jack, who has often defied the authority of the conch previously, does not give Piggy a thought; only the fact that this must mean that "I'm Chief," and he proceeds to attack Ralph. The boys certainly are changed. Roger has seemingly always been brutal and cruel but never revealed such violence until the island. Jack was a respected "Head chorister" and held a position of responsibility before the island. He even reminds them all, early on, that "...we're not savages. We're English" (ch2) when the obeying of rules is discussed. His thirst for power at any cost is only realized on the island and Ralph, a Naval Commander's son, tries to think rationally and do what's best but even he cannot maintain his position and will weep "for the end of innocence"(ch12) because he knows he has been changed for ever.
It is the conflict between Ralph and Jack that contributes to the savagery that descends on the boys as Jack competes with Ralph, despite their "shy liking,"(ch 1) for control of the boys. At first he is content with his "hunters" that Ralph graciously concedes to when he is voted chief and can see Jack's "blush of mortification" but, ultimately, Jack's controlling personality becomes more apparent as he craves complete power and, as Ralph's confidence in his own ability and the likelihood of rescue reduces, so he takes advantage and gains more support. Ralph wants what's best for everyone and Jack just wants to satisfy his own selfishness. Ralph's responsible behavior ensures shelter and fire and all things that will lead to rescue whereas Jack's irresponsible behavior is based on the here-and-now and what he can get out of the situation with no vision of the future.
The Navy rescues the boys and it is significant because it may indicate a return to civilization and a second chance for Ralph. It is war time but the boys will probably be safe from danger as the naval officers will protect them but whether Jack and Roger will be safe from themselves, only the reader's own interpretation can decide that. A romantic reader would say yes that Roger and Jack reform but a more dramatic reader would probably be able to imagine any number of horrible things these boys are capable of. The author then is perhaps allowing the reader to draw his or her own conclusions whether the rescuers, significantly civilized English naval officers, are enough to ensure that the boys conform or whether civilization arrived too late.