Lizzy and Charlotte discuss the consequences of Jane's not showing her feelings towards Bingley. What does this foreshadow?
By not revealing her feelings for Mr. Bingley, Jane gives him and others the impression that she does not have romantic feelings for him. Jane, however, we learn, is just shy and a little bit apprehensive about doing so. She is not as outgoing as Lizzie, for example.
As a result of Jane not showing her feelings, Mr. Darcy gets the impression that she does not care for his friend Mr. Bingley. He relays this opinion to Mr. Bingley, who reluctantly gives up on Jane for a time. Her inability to reveal her feelings foreshadows the fact that Bingley will, for a time, give up on her, and that it will take some intervention to get them "on the same page", so to speak.
Luckily, Mr. Darcy learns from Lizzie that Jane does, in fact, love Mr. Bingley, and as a result, he encourages Mr. Bingley to return and ask for Jane's hand in marriage. He does return and Jane accepts his proposal.
In Ch. 6 Elizabeth remarks to Charlotte her friend how Jane who is admired by Bingley is careful not to make public her feelings of love and admiration for him:
It was generally evident whenever they met, that he did admire her; and to her it was equally evident that Jane was yielding to the preference which she had begun to entertain for him from the first, and was in a way to be very much in love; but she considered with pleasure that it was not likely to be discovered by the world in general, since Jane united with great strength of feeling a composure of temper and a uniform cheerfulness of manner, which would guard her from the suspicions of the impertinent.
Elizabeth feels that Jane is doing the right thing because this would protect her from becoming a victim of the local village gossip.
But Charlotte very perspicaciously remarks that if Jane doesn't reveal it to Bingley very plainly that she loves him and that she will accept him if he proposes marriage then there is the real danger that she might lose him:
it is sometimes a disadvantage to be so very guarded. If a woman conceals her affection with the same skill from the object of it, she may lose the opportunity of fixing him.
Charlotte's statement is proved true when we read Darcy's letter in Ch.35 in which while defending himself against Elizabeth's accusation that he was responsible for preventing Bingley from marrying her sister Jane, he states that on observing Jane and Bingley together he was convinced that Jane was very reserved and that it did not appear to him that she was in love with Bingley:
I observed my friend's behaviour attentively; and I could then perceive that his partiality for Miss Bennet was beyond what I had ever witnessed in him. Your sister I also watched. -- Her look and manners were open, cheerful, and engaging as ever, but without any symptom of peculiar regard, and I remained convinced from the evening's scrutiny, that though she received his attentions with pleasure, she did not invite them by any participation of sentiment.
Thus, the conversation between Elizabeth and Charlotte and Charlotte's astute remarks in Ch.6 foreshadow Bingley's temporary separation from Jane.