The shelters that Ralph tries to build to house the boys on the island have several symbolic meanings. They stand for civilization, altruism, and flawed humanity. The shelters are the home base of the boys and are the priority for Ralph, Simon, and Piggy, the three boys who exhibit the most maturity and dedication to living according to rules and the way they have been raised. In Chapter 3, only Simon and Ralph are left, trying to put up the third hut. Piggy can't help build because of his physical condition, but he says in Chapter 2, "The first thing we ought to have made was shelters down there by the beach."
The shelters also represent altruism. Ralph complains that no one else helps with the shelters other than Simon, even though they all agreed that "everyone was going to work hard until the shelters were finished." But the boys work for about five minutes and then run off "bathing, or eating, or playing." Those who don't help with the shelters are satisfying their own selfish desires, not working for the good of others. Only Simon and Ralph have enough altruism to work at something that won't benefit only them and to delay their own gratification for the good of others. Although hunting would benefit the entire group if the boys killed a pig, Jack and the boys don't hunt to benefit others, but to satisfy their own personal urges.
Finally, the shelters are a symbol of flawed humanity. When Ralph and Simon are trying to build the third hut on their own, "the leaves came apart and fluttered down. ... Ralph surveyed the wreck with distaste." They have been able to erect only two shaky shelters, and "this one was a ruin." Ralph pronounces, "Never get it done." The insufficiency of the shelters foreshadows the doom of the boys' society. Just as Ralph and Simon can't make adequate shelters with the materials they have, so it is impossible to fashion a healthy society from the depraved individuals who are its components, as the rest of the novel will show.
As the previous post pointed out, the huts represent the civilization and the law and order that Ralph hopes can be built up on the island. He is eager to be rescued and sees organization and rules as the way to make that happen. He tries to organize the boys to build the huts, but they fail and end up with only a few miserable structures after hours of labor.
From the very start, Ralph's efforts to organize the boys and maintain order are doomed. Ralph tries over and over again to combat the savagery that exists inside all the boys but ends up failing. Jack manages to see this savagery and their fear of the unknown as the best way to usurp Ralph's power and quickly manipulates the boys to come over to his side.
By chapter 10, when Ralph and Piggy and Samneric are the only boys left on their side of the island, there is some talk of trying to maintain the fire and keeping things going on their side. This, of course, is futile once Jack and the hunters raid and steal Piggy's glasses later that evening.
The huts that Ralph and his crew try to build represent civilization. Ralph is the representative of the civilized life, whereas Jack represents the savage life, and while Jack is successful at hunting, Ralph is unsuccessful at hut building. The huts that Ralph and the others try to build do not withstand the elements effectively and as is shown when Jack's tribe steals Piggy's glasses (thus gaining power through the ability to make fire), the huts are shown to fail at keeping out the savage life. Golding's message in the book was that inside of everyone there is evil in the form of savagery and he shows this, in part, through the failure of the huts. He is telling the reader that people may go through the motions of building civilization, but it's just that - motions. People are still savage and evil inside.