"Foreshadowing" refers to details in the story that suggest the possible outcome, that there is more story to come. Foreshadowing can take the form of a word, a phrase, or broader hints.
In chapter III of Pride and Prejudice, the Bennet sisters, Jane and Elizabeth, attend a local assembly. Mr. Bingley and Mr. Darcy are also in attendance. In Regency society, dances are important sites of social interaction; partners who dance with one another more than once are seen as a potential couple according to the period's social codes. So, when Bingley dances with Jane twice, and his sisters pay her special attention, it signifies that not only has Jane caught Bingley's notice, but his family approves of her also. Jane and Bingley's positive social interaction foreshadows the possibility of their becoming a couple.
Jane and Bingley's easy interaction contrasts sharply with Elizabeth and Darcy's. While the novel focuses upon Elizabeth and Darcy's evolving relationship, in Chapter III, their interactions are disastrous: Darcy insults Elizabeth, and Elizabeth retells the story to her friends and family and pokes fun at what seems to be his well-developed ego. This isn't exactly well-mannered either, so both are at fault in this scene.
The chapter ends with Mrs. Bennet speaking of how "horrid" Darcy is and how much she detests him. This strong emotion, expressed so early in the novel by both mother and daughter (Elizabeth) suggests strongly that the narrative will soon take a turn and force a re-evaluation of Mr. Darcy's character. (This re-evaluation will, as domestic narratives rely upon symmetry, be accompanied by Darcy's re-evaluation of Elizabeth too.)