It just means that Jack and the hunters are going to kill Ralph, cut off his head, and offer it as a sacrifice to the Beast (which is nothing but a dead paratrooper). One sharp end goes into the ground, the other sharpened end will hold Ralph's head. With Ralph gone, the island will be totally ruled by Jack and Roger and what little civilization is left will crumble. Sam and Eric have been captured now and will have to do whatever they're told.
The reference in chapter 12 to the "stick sharpened at both ends" comes from the hushed conversation Ralph has with Samneric when Ralph is hiding from Jack and all the other boys. Ralph is being hunted, just like a pig, and he is a tribe of one with all the others on Jack's side now, either by choice or by coersion. Samneric tell Ralph that Roger sharpened a stick at both ends because Roger means to put one sharpened end into the ground and put Ralph's severed head on the other sharpened end of the stick. Ralph would be regarded in the same way the boys regarded the pig whose head they earlier stuck on a stick that was sharpened at both ends. That particular pig was treated that way as a sacrifice to the fictional "beast" that the boys in Jack's group feared. That head became the Lord of the Flies that Simon, and later in chapter 12, seemingly spoke and identified the real evil on the island as the savagery inside each person. By reducing Ralph to nothing more than a pig, the boys have reached complete savagery.
It can hurt the hunter as well as the prey, and so symbolises the fact that the society that Jack has created is self-destructive. Get a text revision guide or something :p
as mentioned before... the stick sharpened at both ends is a reference to when jack's tribe killed the sow.. but if you want to go to a more interpretive level... some say the stick sharpened at both ends represents death and inanimate representation of the depths to which savagery will lead, again, death, murder and sadism. You can also say that the stick shows a no-win situation, kill or be killed, where the fight for survival reaches a desperation where all the intelligence we humans pride ourselves on disappears into primal instincts.
The stick sharpened at both ends isn't much referred to in the novel until nearer the end, as the boys complete their descent into savagery, however, if writing an essay you can say that as the boys get more savage, the spears they use become sharper and used with more muderous intent, until they reach a pinnacle point where it is not only sharpened at one end, but two.
hope that helps :) obviously there will be somewho disagree with me, but that is my interpretation.. (sorry about spelling mistakes)