Pride and Prejudice is about gentry-class people. There are a few aristocrats—people with titles, such as Lady Catherine de Bourgh—but the action primarily involves people a rung or two below the aristocracy. Mr. Bennet, for example, is a landed gentleman with an estate that brings in a good income and enough property to take guests hunting, but he has no title. Mr. Collins is a clergyman who will inherit a landed estate; he is in a respectable position but still of the gentry class. Likewise, most of the people the Bennets associate with, and even the Bingleys, are gentry-class. The Bingleys are rich, but they are not aristocratic.
This social background is important for several reasons. First, Austen is not at all concerned with working-class or poor people in the novel. The gentry are a class comfortably well off, but at the same time, they often have constraints: the Bennet girls, for instance, have no dowries, which was money that women would bring to a marriage to offset the expense of marrying them. Charlotte Lucas is in the same position. Marriage is of prime importance to women in this situation, while made more difficult by it, because they cannot expect to inherit enough to live on and yet, because of their class, cannot expect to work for a living. Even were they to become governesses, one of the few work options open to gentry-class women, this would be considered falling down the class ladder because they would be earning money.
As the novel shows, gentry women are under extreme pressure to marry, but obstacles are put in their paths. For example, Lady Catherine's snobbery is affronted that someone like Elizabeth would dare to marry Darcy, and she tries, ineffectually, to block the marriage.