How does Ralph "assert his chieftainship" ?
This is a reference to a specific moment in Chapter 4, where Ralph is said to have inadvertently "asserted his chieftainship" better through casual passive aggression than if he had thought about an alternative plan for days.
The conflict over which Ralph needs to assert himself is the fact that the hunters allowed the signal fire to go out, inadvertently robbing the boys of a chance to be noticed by a passing ship. Jack offers all manner of excuses, insisting that every hunter was needed for the pig hunt, and failing to truly realize or appreciate the fact that the immediate gratification provided by the pig is irrelevant when compared to the need for getting off the island. Jack then "formally" apologizes, which garners him support from the hunters, but which Ralph recognizes as a thinly-veiled attempt at seizing power from Ralph.
Ralph needs to make the hunters' failure plain to them, but he can't waste any more words, or physically punish them. Instead, he orders that the fire be rebuilt, but he stands in the place where the old one was, refusing to move. This effectively forces the hunters to be inconvenienced by building the fire in a new, slightly worse place, while retaining the "grave" of the first fire as a reminder of their error.