Under the Dome of what, exactly, the reader might ask. Throughout the novel, characters are stumped about the dome that has randomly appeared over their town. That leaves them isolated from the broader community, which means this novel is at least partially an examination of community, because only the immediate community of Chester's Mill suffers the problem. The government is no help, nor is the military, so in a way, although the dome is the main feature of the plot, the novel is really about isolation and community.
Look at the drug problem that arises in the novel. Methamphetamines were kind of a problem before, but since the dome appeared, the drug problem has become one of the strongest economies. That is like a symbolic reminder that drugs and crime constitute a vast majority of public concern, especially in a community. Also, the dome represents a kind of enforced stand-still that makes the town jittery and more like to abuse drugs, because they're nervous, confused, horrified, and mostly they are paranoid. So, the drug problem is essentially an issue of isolation, mental imbalance, and fear, at least in the novel's point of view.
That makes sense, and it helps to understand the way the dome effects the broader community, because although many people in the dome do not use drugs, they are forced to admit that their broken defense mechanism are not sufficient for dealing with a problem as universal and unexplainable as the dome. That is a good indication of the dome's true meaning as a symbol. Yes, it represents the insulation of a community, but what is the real insulation around communities in real life? It is the limitation of one's power.
Because the people of Chester's Mill are unable to move, they are limited to their situation perfectly. That symbolizes the lack of social mobility, because it is a limitation on the literal movement of literal people. King's novel shows that when people cannot move from one station to another, they will become panicked and frantic, and addictive behavior will sky-rocket. Although there is no real dome over any real city, the limitations of small town life often feel as if there is a dome.
