Symbolism, Imagery, Allegory
Symbols are signs (or representations) of other things, so what happens when you have Signs that are symbols? Well, first you get a little dizzy trying to wrap your head around that last sentence, and then you keep reading so we can sort this all out.
When Will goes on his quest to defeat the Dark, he's told to collect six Signs. They all look the same—a circle with a cross in the middle—but each is made from a different material. Will is told the following poem to help him remember what he's supposed to do:
When the Dark comes rising, six shall turn it back; Three from the circle, three from the track; Wood, bronze, iron; water, fire, stone; Five will return, and one go alone. (3.61)
Back in the day, the Signs were made of different materials to represent different elements on the earth. Did you notice how these elements make up pretty much everything in the natural world? Because they do. This enables the Old Ones to keep the Signs safe from the Dark by hiding them in different places throughout the world. Cool trick, and a reminder that the stakes are so high that secrecy is in order. That the Signs are made from natural elements is also a subtle shout-out to the fact that the world—where natural elements come from—is in jeopardy.
We can see in the poem that six people (six Old Ones) will have the power to stop the Dark from rising; they "shall turn it back." So as much as the Signs are about defeating the Dark, in Will's quest for them the Signs are also representative of his immersion into the Old Ones community (not to be confused with an old folks' home). As he gathers the Signs, he meets different Old Ones, gaining a sense of who the Old Ones are and his own identity, as well as becoming more invested in his mission to defeat the Dark. Check it out:
He thought: these are my people. This is my family, in the same way as my real family. The Old Ones. Every one of us is linked, for the greatest purpose in the world. (13.33)
By the end, it's clear that the Signs haven't just helped the Old Ones defeat the Dark—they've helped Will really connect with this part of himself. As the Old Ones celebrate their success, Will sees them as "family," which is no small statement given how fond he is of his own blood relations.