Apparently, Number Twelve, Grimmauld Place, was Sirius's parents house.
Now that they're dead, it belongs to Sirius.
Sirius adds, "I offered it to Dumbledore for Headquarters — about the only useful thing I've been able to do" (5.4).
Harry is surprised by how bitter Sirius sounds.
In the basement kitchen, Mr. Weasley and Bill greet Harry.
Mundungus Fletcher is also there. He admits to Harry, "Owe you a 'pology" (5.24). He adds: "See, I wouldn't 'ave left [...] but I 'ad a business opportunity —" (5.36).
Sirius tells Harry he'd welcome a Dementor attack around now.
Sirius has been stuck inside for a whole month.
Dumbledore hasn't let Sirius go anywhere or do anything.
It's too risky since Sirius is a wanted criminal (the Ministry of Magic think he's a Voldemort supporter who killed Peter Pettigrew fourteen years ago). What's more, now it seems like Voldemort knows all about Sirius's Animagus form (he transforms into a giant black dog). So, Sirius can't disguise himself that way anymore.
All Sirius can do is help try, "to make this place fit for human habitation [...] No one's lived here for ten years, not since my dear mother died, unless you count her old house-elf, and he's gone round the twist — hasn't cleaned anything in ages" (5.51).
Mr. Weasley, Bill, and Lupin are all talking about whether or not the goblins will join Voldemort.
Lupin cautions: "I think it depends what they're offered [...] And I'm not talking about gold. If they're offered the freedoms we've been denying them for centuries they're going to be tempted" (5.76).
Meanwhile, Mundungus is telling the table about his shady business deals.
Sirius explains that Mundungus is in the order because he's "useful [...] Knows all the crooks — well, he would, seeing as he's one himself. But he's also very loyal to Dumbledore, who helped him out of a tight spot once" (5.86).
Mrs. Weasley tries to send the kids off to bed, but Sirius wants to talk about Voldemort first.
Mrs. Weasley throws a fit; she and Sirius get into a nasty argument over what Harry (and Fred, George, Ron, and Ginny) is old enough to know.
Sirius argues, "I don't intend to tell him more than he needs to know [...] But he was the one who saw Voldemort come back" (5.104).
And Mrs. Weasley replies, "He's not a member of the Order of the Phoenix [...] He's only fifteen and —" (5.105).
Lupin provides the voice of reason, "I think it better that Harry gets the facts — not all the facts, Molly, but the general picture — from us, rather than a garbled version from ... others" (5.121).
(Harry is sure Lupin knows about the Extendable Ears.)
So finally, Harry (and Fred, George, Ron and Hermione but not Ginny, who's the youngest) gets his news.
Voldemort is lying low.
While he has succeeded in coming back to life, his return was ruined by Harry's survival.
Sirius tells Harry, "Nobody apart from his Death Eaters was supposed to know he'd come back. But you survived to bear witness" (5.160).
Harry's warning gave Dumbledore the chance to call up the Order of the Phoenix.
Voldemort wants to build up his army again, so the Order has been trying to stop Voldemort from getting more followers.
The Order's efforts are made harder by the Ministry of Magic. Cornelius Fudge is "absolutely refusing to believe" (5.176) Voldemort has come back at all.
Fudge worries that, if Voldemort comes back, Dumbledore will become the next Minister for Magic.
So, to hold on to his power, Fudge is working hard to make both Dumbledore and Harry look unreliable and untruthful. He doesn't want anyone to believe their stories about Voldemort's return.
Lupin confirms, "You see the problem [...] While the Ministry insists there is nothing to fear from Voldemort it's hard to convince people he's back, especially as they really don't want to believe it in the first place. What's more, the Ministry's leaning heavily on the Daily Prophet not to report any of what they're calling Dumbledore's rumor-mongering, so most of the wizarding community are completely unaware anything's happened" (5.187).
The Order is trying to get the word out, but it's hard: Lupin is a werewolf, so he's on the margins of wizarding society.
And Sirius is still wanted for murder, so he's not exactly gaining the public's trust.
Tonks and Mr. Weasley would lose their jobs at the Ministry if they started speaking up too much.
Mr. Weasley worries, "If [Dumbledore] carries on defying the Ministry like this he could end up in Azkaban, and the last thing we want is to have Dumbledore locked up" (5.199).
Voldemort is also trying to put together a weapon he didn't have before.
At this news, Mrs. Weasley shouts, "That's enough! [...] I want you in bed, now. All of you" (5.208-10).