The fire is important to Ralph because he knows it is their one chance for rescue. Without its smoke as a signal, no ships or planes could ever known the boys are on the island. There's no cell phones, no internet...no one even knows if the island has actually been charted. So for Ralph, the fire symbolizes the one hope to return to civilization and order. It also represents responsibility, as it is Ralph's one decision/order as chief: to keep the fire going. Because it marks an attempt to reconnect with the original society, Ralph is very attached to it, & this affects his interactions with the the other boys. In a sense, it drives a wedge between he & Jack. This makes it even more important for Ralph to make the others understand the significance of the fire.
The conch shell was also very important to Ralph. At first, it serves as a symbol of order and democracy. The boys come when Ralph blows the shell, and they follow the rules established by such (at least for the first meeting). Yet that quickly disintegrates, as by the second meeting, boys are talking over each other, taking the shell from each other, and even resorting to violence. The shell, much like the fire, represents a desire to lead through order and civil nature, as opposed to savagery and threat. This does not work however, and eventually the shell will be crushed, like Ralph attempt at democracy.
To a lesser extent, Piggy and his glasses are important to Ralph, for many of the same reasons stated above. Piggy's glasses symbolize insight and invention, & again, are a link to the civilization from which the boys came. It is not until the end that Ralph understands how important Piggy's friendship was, and mourns his death as that of "a true friend."