Lady Catherine actually arrives in her coach to dictate to Elizabeth what she should do. She has heard a rumor that Elizabeth and Darcy are engaged and takes it upon herself to forbid the marriage, saying Elizabeth must back out because she is not a suitable bride for Darcy. Further, Lady Catherine says, it has always been intended that Darcy marry her own daughter.
As we know by now, Elizabeth has a temper and is not one to take insults lying down. She informs Lady de Bourgh in no uncertain terms that she is a gentleman's daughter and thus as suitable as Lady Catherine's own daughter to marry Mr. Darcy. Further, she tells Lady Catherine that she has no right to interfere in her marriage and that both she and Darcy can marry as they choose.
This is quite the affront to Lady Catherine, who feels she has the authority to tell everyone what to do all the time and be obeyed. Nevertheless, we learn that after Elizabeth and Darcy wed, Lady Catherine reconciles herself to Elizabeth in order to be invited to Pemberley.