Piggy is the much-hated, overweight boy on the island who represents exactly what the boys are turning away from: rational thinking. It is Piggy who determines that they need to count the number of boys on the island. It is Piggy who decides to use the conch to assemble the boys. It is Piggy who wants to learn everyone's name. It is Piggy who tells the group that the beast can't be real. And it is Piggy who notes that the signal fire should be moved to increase their chances of survival.

Piggy's ideas aren't valued because of his weight and physical challenges; the other boys find him pathetic and ridiculous, never to be taken seriously. Because Jack successfully leads the group of boys increasingly toward utter savagery, the boys display a growing sense of irritation and even anger toward rational thinking. Instead, they react emotionally and in a mob mentality.

In Chapter 8, notice how the pigs Jack hunts are described:

The pigs lay, bloated bags of fat, sensuously enjoying the shadows under the trees. There was no wind and they were unsuspicious; and practice had made Jack silent as the shadows.

Compare this to the final scene when Piggy is alive in Chapter 11:

Someone was throwing stones: Roger was dropping them, his one hand still on the lever. Below him, Ralph was a shock of hair and Piggy a bag of fat.

(Bold added for emphasis.)

It is in this scene that Piggy tries to tell Jack's group that living like savages is irrational. He tries to convince them to act more maturely and to follow the rules in order to be rescued.

Rescue isn't what this group wants, and Piggy's sense of rational thinking is not needed. Therefore, the group moves from killing the pigs to killing Piggy and all that he represents.

Piggy acts as a very interesting symbol in Lord of the Flies. Previous answers to this question have stated that the boy represents logic, the rational world/civilization and that his glasses in particular represent the boys trapped on the island not being able to look ahead and see the consequences of their actions. 

Let's take things a step further in the area of Piggy's glasses. Unlike the other boys, Piggy CAN see (figuratively speaking), and his glasses allow him to do so. Piggy himself represents logic and scientific thinking, and his glasses both literally and figuratively represent advancement in science and technology. They are quite literally a tool, an advancement, used to help Piggy be able to see with his eyes. When they are used to create fire, they become a representation of the human capability to invent and innovate. While still somewhat naive to the chaotic ways of the island, Piggy has insight into cause and effect, and he has a desire to keep a sense of order— his glasses are a literary device used to emphasize this. 

Piggy is the overweight, intelligent boy with asthma throughout the novel Lord of the Flies, whose character symbolizes the rational world and civilization. Unlike the other boys on the island, Piggy is physically vulnerable and relies on the laws and regulations of civilization to protect himself, which is why he remains close to Ralph. However, Piggy is by far the most intelligent boy on the island and pragmatically attempts to solve issues using his intellect. He views life as scientific and offers several solutions to pressing issues on the island. Unlike the other boys, Piggy understands the importance of a signal fire, obeying the conch, and building shelters. Piggy is also the strongest proponent of civility on the island and cannot comprehend Jack's savage nature. Essentially, Piggy symbolizes the rational world and civilization because of his intelligence, affinity for the conch, enmity towards Jack, and pragmatic disposition. 

Lord of the Flies is an allegorical novel and, as such, the characters represent important ideas or themes. Piggy signifies logic and has a scientific intellect. Piggy is sensible and rational and he tries to think how 'grown-ups' and, in particular, his 'auntie' would behave in difficult situations.

He represents reason and loyalty.

What is also highly symbolic is the fact that Piggy cannot see well - hence his glasses. Piggy is used throughout the novel to show the apparent shortsightedness of the boys (and therefore society in general) when they do not consider the consequences of their actions. Whilst Piggy has the ability to be

calm and thoughtful about situations presented, the others are playing dangerous games that culminate in(to) murder and evil.

Piggy's scientific thinking reflects the development of science and its importance far beyond the scope of the novel.

Refer to the eNotes study guide and navigate to the character analyses and other useful information to a gain a better understanding of what the characters represent in Lord of the Flies.