Suburbia means the suburbs taken all together — not one particular suburb, but the whole world of them, and, more than that, the way of life that goes with them. When somebody says suburbia they are usually thinking of rows of similar houses, quiet streets, gardens, shopping malls, families with children, cars in every driveway, and a certain calm sameness.

It is an uncountable noun and takes no article: people say "life in suburbia," never "a suburbia" or "the suburbia." That is worth noticing, because it behaves more like the name of a country than like an ordinary noun — which is exactly the joke inside the word. It treats the suburbs as if they were one imaginary place you could move to.

The feeling attached to it is usually mild disapproval: safe, comfortable, respectable, and rather boring, with everyone watching what the neighbours think. Novels, films and songs have used it that way for decades, and phrases like "the boredom of suburbia" or "escape from suburbia" are very common. But it is not always an insult. Writers also use it plainly for a real social fact — "suburbia grew rapidly after the war" — and some people say it warmly, with affection for a quiet childhood. You have to listen to the sentence around it to know which one you are getting.