"If I can but see one of my daughters happily settled..."
See in text (Chapter III)
Mrs. Bennet’s wish to see one of her daughters marry Mr. Bingley is comically unrealistic. A man of wealth and status would have been unlikely to marry a woman from a poor (or at least not solidly well-off) family. Mrs. Bennet’s somewhat ridiculous ambitions suggest that Austen is developing a satirical rendition of the classic Marriage Plot.
"When she is secure of him..."
See in text (Chapter VI)
Charlotte Lucas does not seem interested in Elizabeth’s romantic ideals. She thinks the risk of becoming a spinster outweighs the need for love and happiness; Jane should therefore “secure” Mr. Bingley’s proposal before she loses all hope of marriage.
See in text (Chapter VII)
An entail legally restricts the sale or inheritance of an estate. Longbourn has been left (entailed) to a male heir, so Mr. Bennet can’t leave it to his daughters. When he dies, all of the Bennet women will be poor.
See in text (Chapter XIII)
Mrs. Bennet refuses to accept that Longbourn cannot be sold or left to anyone other than Mr. Collins. An entail is a legal restriction on the sale of an estate; in Austen's time, it guaranteed that family estates remained intact through the male line of succession. An entail essentially prevented men from bequeathing their estates upon illegitimate or step-children (neither of which are technically of the family bloodline). Mr. Bennet therefore has no control over whether or not Mr. Collins inherits Longbourn. Austen uses this entail as one of the primary bases of the plot; Mrs. Bennet's obsession with matchmaking originates in it.
"may turn you all out of this house..."
See in text (Chapter XIII)
The Bennets' estate is entailed to another person. When Mr. Bennet dies, the rightful heir will arrive to claim his inheritance and the Bennet women will have to move. Austen uses the entailment of the estate as a primary basis of the plot; it is the reason Mrs. Bennet is obsessed with matchmaking and so forcefully pushes Jane and Mr. Bingley together.
"it was impossible for that friend to be tolerably happy in the lot she had chosen...."
See in text (Chapter XXII)
Elizabeth is upset because Charlotte has chosen security over love and happiness. Charlotte and Elizabeth have different ideas about matrimony; Charlotte sees marriage as a pragmatic necessity, and Elizabeth can't imagine being married to someone who makes her unhappy. The reality, however, is that Elizabeth (like Charlotte) will be destitute if she doesn't marry well. Longbourn is entailed to Mr. Collins, so he will inherit the estate when Mr. Bennet dies and the Bennet girls won't receive anything.
See in text (Chapter LVIII)
Though Mr. Darcy's proposal (and Elizabeth's acceptance) may seem lackluster after hundreds of pages of build-up, the union between Elizabeth and Mr. Darcy is nearly inevitable at this point. Mr. Darcy's intervention on Lydia's behalf confirms his devotion to Elizabeth, and Elizabeth's vastly-changed opinion of Mr. Darcy strongly implies that she will be receptive to a second proposal.