Ralph uses the issue of the fire going out to demonstrate his power by using the opportunity to make clear that his decisions are for the good of the group—and that Jack's bloodthirsty escapades are reckless.
When Jack returns from the hunt, he and his group don't notice at first that Ralph and the others who are waiting for them are upset. Instead, Jack's group breaks into excited details about the methods they used to kill a pig. Ralph allows them to carry on for a bit before calmly explaining that they had been responsible for the fire, which is now dead. Jack makes excuses and points out that the fire had only been out for "an hour or two."
Ralph keeps his voice steady as he replies, "There was a ship." His calm and rational demeanor is a sharp contrast to Jack's boisterous energy. In this moment, it becomes clear that Jack's failure to adhere to his responsibility has cost them an opportunity to be rescued. Jack's vision for the group is short-sighted, and it is clear in this moment that he is not the best leader. He has utterly failed them at the point when they needed him most.
Ralph thus emerges as a trusted leader, and Jack's excuses don't hold up as he complains that the hunting job was "too much" and that he "needed everyone." Calmly, Ralph demands that Jack "light the fire." Jack is forced to submit, and when Ralph refuses to move from the site of the old fire, Jack relocates it to a different spot. Because Jack now seems to be an incompetent choice of a leader, Ralph is able to assert his own chieftainship in a way he has never been able to. Jack's disregard for the needs of the group allows Ralph to emerge as the leader the group needs.