Before answering this question, it helps to make sure we know what free indirect discourse is. Free indirect discourse is a special kind of third person narration that combines elements of third person and first person narratives. In other words, free indirect discourse has access to a character's inner consciousness, but delivers this knowledge in a third person format. As such, with free indirect discourse, the narrator will report the thoughts and/or inner feelings of a character, but will simply say them, rather than directly attributing them to the character. Thus, with free indirect discourse, we get an indirect access to a character's consciousness. James Joyce and Jane Austen are notable authors who use free indirect discourse, although they approach the technique in different ways.
In Jane Austen's Pride and Prejudice, Austen uses this technique on several occasions, and she sometimes does so in order to mislead us, as we're drawn into thinking the thoughts and feelings of a particular character are actually objective judgements coming from the narrator. Take, for instance, this passage from Book/Volume 1, Chapter 16 discussing Elizabeth's opinions of Mr. Wickham (the quote is taken from eNotes' excellent online version of the text):
Elizabeth allowed that he had given a very rational account of it, and they continued talking together with mutual satisfaction till supper put an end to cards, and gave the rest of the ladies their share of Mr. Wickham's attentions. There could be no conversation in the noise of Mrs. Philips's supper party, but his manners recommended him to everybody. Whatever he said was well said; and whatever he did, done gracefully. Elizabeth went away with her head full of him.
At first glance, this passage seems to be a conventional passage coming from the narrator. However, if we look closer at it, we see that much of it is actually coming from Elizabeth's inner thoughts and feelings, specifically her inner thoughts and feelings in regards to Mr. Wickham. As the latter half of the passage suggests, Elizabeth is becoming quite infatuated with Mr. Wickham and believes him to be an upstanding gentleman. Moreover, since this passage comes soon after Mr. Wickham's explanation of Mr. Darcy's "wrongs" against him, it shows us that Elizabeth is quite ready to believe everything he says. This use of free indirect discourse is very misleading, in the end. Mr. Wickham proves to be a scoundrel and Mr. Darcy is, in fact, an honorable (if overly proud) man. Thus, Elizabeth's initial feelings about Wickham are misleading, especially since they're delivered through free indirect discourse and seem like objective fact.