Sirius tells Harry that Mrs. Black must've put a Permanent Sticking Charm on the back of her portrait, so they can’t take it down. He says that this was his parents' house. They enter the basement kitchen, which is gloomy and littered with chairs and empty wine bottles in the aftermath of the Order’s meeting. Mr. Weasley and Bill greet Harry as Bill rolls up parchment that seems to contain floor plans of a building. When she sees Harry looking, Mrs. Weasley snaps that they need to clean up immediately after meetings. Sirius introduces Harry to Mundungus, who begins to awkwardly apologize.
It's presented as a surprise that this is Sirius’s childhood home, since it seems like such a gloomy and even evil place. The floor plans are a motif that show up often in heist or action films, and used here, they likely signal to Harry that the Order is doing something exciting and meaningful with them. Mrs. Weasley's anger that Harry saw them suggests that she believes it's extremely important to censor what information Harry gets and keep him in the dark, likely believing it safer to do so.
Everyone but Sirius, Mundungus, and Harry help Mrs. Weasley prepare dinner. At the table, Sirius tells Harry that Harry's summer sounds delightful, as he's been stuck inside for the last month. Sirius sounds just as upset with Dumbledore as Harry is. He says that Snape keeps making snide comments about Sirius spending time safely at home, cleaning. Suddenly, Mrs. Weasley shrieks and Harry sees most of dinner, complete with a knife, flying toward the table. The knife lands right where Sirius's hand just was and Mrs. Weasley screams at Fred and George, who are responsible for this, while Harry and Sirius laugh.
Remember that Fred and George are now adults in Wizarding society, which is why they can Apparate and do magic like this outside of school. Just as with Tonks's admittance that she's not good at some things, the way the twins are seemingly struggling to ascertain how to appropriately behave as adults in the Wizarding world suggests that there's a learning curve as wizards come of age. The twins are also just characteristically reckless and willing to take risks for the sake of a joke.
Over dinner, Mrs. Weasley mentions to Sirius that there's a boggart trapped in the drawing room and doxies in the curtains, both of which she'd like to tackle tomorrow. Mr. Weasley, Bill, and Lupin discuss whether the goblins might join Voldemort. Mundungus, meanwhile, tells Fred, George, and Ron about selling someone their own stolen toads, which offends Mrs. Weasley. Sirius tells Harry that Mundungus is a useful person to have around, as he knows all the criminals and is loyal to Dumbledore.
The existence of the boggart shows the reader that what Harry's learning in school is actually useful for the real world (Harry's class learned to do away with boggarts in their third year). That lesson was a practical one, which suggests that there's value in actually practicing magic, something that will become important later.
After dessert, Mrs. Weasley tries to send everyone to bed, but Sirius suggests they speak to Harry and let him ask some questions. Sirius and Mrs. Weasley argue; Sirius believes Harry has a right to know things, but Mrs. Weasley thinks Harry is too young. She points out that Harry isn't James and tells Sirius that he can't treat Harry like James. Mr. Weasley and Lupin quietly suggest that they fill Harry in, if only so he doesn't get partial facts from others. Lupin says that Harry's old enough to decide if he wants to know or not. Harry feels bad for going against Mrs. Weasley, but he says he wants to know. Mrs. Weasley then tries to send everyone else to bed, but only successfully sends Ginny to bed.
Remember that James and Sirius were best friends in school. Mrs. Weasley's admonition then suggests that Harry and Sirius's relationship is (at least on Sirius's end) becoming more of one between peers than a guardian and a minor child. The series positions the friendships between Harry, Ron, and Hermione as more important than anything else, however, which suggests that the relationship between Sirius and Harry will now be equally important. At the same time, Mrs. Weasley clearly worries that Sirius is trying to relive his youth through Harry, subconsciously pretending that Harry really is James, Sirius’s brave and reckless best friend.
Sirius and Lupin explain that Voldemort is laying low and isn't killing people, and Harry managed to thwart Voldemort's first plans by immediately alerting Dumbledore of his return. The Order has mostly been trying to convince people that Voldemort is back, but they're struggling since Fudge refuses to believe the truth. Mr. Weasley explains that Fudge is afraid of Dumbledore because he believes that Dumbledore wants to be Minister of Magic, so he's trying to discredit Dumbledore and Harry. This makes laypeople easy targets for the Death Eaters, since nobody wants to believe Voldemort's back.
The way that Mr. Weasley connects a lack of information with vulnerability makes the case that holding information can mean power and safety. Fudge's choices here, and specifically his distrust of Dumbledore suggest that Harry can expect some changes at Hogwarts, especially given how much free rein Dumbledore has had thus far to conduct school business as he pleases.
Harry asks what else Voldemort is after, and Sirius cagily says that it's something Voldemort "can only get by stealth," like a weapon. Harry latches onto this and asks what the weapon is like, but Mrs. Weasley enters the kitchen and insists that this is enough. Lupin agrees with her and slowly, everyone stands up and heads upstairs.
Harry latches onto the word "weapon" and seems less interested in the grunt work of quietly spreading the word of Voldemort's return. This again shows that he expects the Order to be guarding something important and exciting.