The next morning, Mr. Bennet reveals to his family that they will have a surprise guest: Mr. Collins, the relative who will inherit Mr. Bennet's estate. The news upsets Mrs. Bennet because Mr. Collins can legally kick Mrs. Bennet and her daughters out of the house when Mr. Bennet dies. But the tone of reconciliation in Mr. Collins' letter consoles her.
Collins is Mr. Bennet's heir because women weren't allowed to inherit. This explains Mrs. Bennet's obsession about getting her daughters married. It was the only way to ensure their financial security, and her own if her husband died before she did.
The letter explains that Mr. Collins is now a parish rector and enjoys the patronage of Lady Catherine De Bourgh—whose wealth and generosity Mr. Collins tirelessly compliments. He now seeks to make peace in the family by some unspecified plan.
Mr. Collins gets ahead in the world not through his own virtues, but by sucking up to the rich and by his almost arbitrary future inheritance of the Bennet's property.
When Mr. Collins arrives, he is heavy, pompous, and dull. His conversation is weighted with overwrought compliments and vague hints about making amends to the Bennet daughters.
Austen makes Mr. Collins a ridiculous comic figure, in the process mocking all suck-ups.