Ralph seems to have a sort of preoccupation with the signal fire. At times it even seems a bit irrational to the others, especially Jack, who begins to minimize or ignore the importance of any effort to be rescued.
I see the signal fire as a symbol of hope, for, as we know from biblical references, "without hope the people perish." Ralph makes many references as to what might happen if the signal fire goes out, all of which symbolically relate to the demise of their "civilization" should the fire, or hope, be extinguished.
I agree with the previous post by pohnpei concerning the fire representing civilized life in Lord of the Flies. But the fire also represents something much more concrete concerning their possible rescue. Ralph sees better than any of the other boys that the greatest likelihood of ever being found is from the smoke emitted by the fire. As long as the fire is burning and creating a smoke trail, then the greater the chance of a passing ship seeing it and coming to their aid. If the fire goes out, there is no smoke, and the chances of rescue dwindle, as do the boys' chances of survival. In the end, it was the smoke which led to their rescue.
I believe that Ralph is using the fire as a symbol of civilization and the desire to get back home. If that is so, then what he is saying is that the boys should die rather than give up on being civilized and on wanting to get back to civilization.
This makes sense for Ralph to say -- after all, he is the character who is most closely connected to the idea of order and civilization. It also makes sense when put together with his other concerns in the chapter. He is afraid their "society" is falling apart and moving more towards savagery.