THe conch represented power and voices being heard since the only person who could talk was whoever had the conch.
Piggy's broken glasses were a symbol of the broken civilization and how the boys were tuning into evil creatures.
As shown below as well, the signal fire represented survival. The boys thought this was the only was they would be saved and for their whole time on the Island, Ralph always kept the signal fire going. At the end, Jack sets the island on fire, trying to kill Ralph and that leads to their rescue.
The "beast" was a major symbol in "Lord of the Flies" because the entire book, the boys thought a beast was on the island. In reality, they were their own beasts in their heads.