In Chapter 1 of Jane Austen's Pride and Prejudice we find a powerful and realistic fact that pertains entirely to the main theme of the novel, and which also sets the tone immediately:
"It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife."
"Tone" in terms of a literary technique, refers to the overall attitude of the author and the characters about the main theme of the story.
In the case of Pride and Prejudice, the statement above represents a tone of sarcasm and irony. Austen puts this statement forward for us to have a clear idea of what the novel contends: That a man who has ways and means is meant to get a wife as part of a checklist of milestones that he should fulfil for society's sake. Inversely, woman is the possession of a man. She is one of the boxes to be checked off a list of milestone that a man is supposed to fulfil.
However, we will find through the novel's heroine, Elizabeth, that not every woman is willing to accept the premise of a marriage for convenience. This shows that the overall attitude of the main character will be disdain, sarcasm, and aversion towards this topic, thus setting the tone of the novel from the very beginning.