If some adult has ever snapped at you to be "civilized," then you probably know that "civilized" ends up referring to all kinds of arbitrary things: get your elbows off the table; say "please" and "thank you"; don't chew with your mouth open; don't flick spitballs at your sister. You know. Being "civilized" usually means not doing what comes naturally. At least, that's how Lord of the Flies seems to see it. What comes naturally is running around slaughtering pigs in war paint; and what's civilized is having names, addresses, meetings, and elected leaders. But those arbitrary markings of civilization might be the only things that make life worth living.
In Lord of the Flies, civilization is arbitrary but necessary; it's the only thing keeping us all from killing each other.
Golding suggests that civilization is ultimately doomed to fail, because the beast in all of us will eventually break free.