Social class is a prominent theme in Austen's Pride and Prejudice in two ways.
First of all, social class is used to certain by characters to represent their inflated sense of self and superiority. In Chapter Eight, for example, the Bingley sisters mock Elizabeth and Jane Bennett because of their "low connections," specifically their attorney uncle who lives in a middle-class area in London. Similarly, in Chapter Nine, Mrs Bennett uses social class in an attempt to impress those who are economically superior to herself: "I believe there are few neighbourhoods larger. I know we dine with four-and-twenty families."
Secondly, social class is a major source of conflict in Pride and Prejudice. Take, for example, Darcy's proposal of marriage in Chapter 34. Any sense of romance is quickly dashed when Darcy dwells on Elizabeth's inferior social position:
His sense of her inferiority, of its being a degradation, of the family obstacles which judgment had always opposed to inclination, were dwelt on with a warmth which seemed due to the consequence he was wounding, but was very unlikely to recommend his suit.
By attacking her social class in such a manner, Darcy causes great offence to Elizabeth and this is one of the reasons why she spurns his marriage proposal. Austen then uses this conflict to drive the plot of the novel. But, ultimately, through Elizabeth's (eventual) acceptance of Darcy, and through Jane's marriage to Mr Bingley, Austen shows that love can overcome any obstacle, even the restrictive boundaries of social class.