What It Is
Apartment number 50 in the building on Sadovaya Street, in Moscow, where Stiopa Likhodeyev wakes up hungover and where Berlioz's vacant rooms are. It already had a sinister reputation before the story, because earlier residents vanished without a trace. After Berlioz's death it becomes the headquarters of Woland's retinue, hosts Satan's ball, and ends up burned in chapter 27.
Where It Appears
In chapter 7, Woland introduces himself to Stiopa, seated beside his bed, and the retinue decrees that Stiopa is surplus to the apartment; he wakes up on an embankment, in another city.
Master of black magic, Woland
Yalta, come now...
In chapter 9, thirty-two complaints from neighbors contest Berlioz's rooms, and Korovyov rents the whole apartment for Woland from Nikanor Ivanovich Bosoy, the chairman of the residents' association; Nikanor is arrested shortly after, at home.
What does it mean to be an official or an unofficial representative? It all depends on the point of view from which you look at the object. All of it, Nikanor Ivanovich, is relative and unstable.
In chapter 10, Varenukha is dragged there by Azazello and Behemoth, and Hella intercepts him in the vestibule.
Let me give you a little kiss
In chapter 12, already at the Variety Theatre, Woland gives his verdict on the Muscovites.
They are people like any others. They like money, but it has always been so... Humanity likes money, no matter what it is made of: leather, paper, bronze, or gold. Well, they are frivolous... what can you do... mercy sometimes strikes their hearts... they are ordinary people... on the whole they remind one of people from the past... only the housing problem has corrupted them...
In chapter 18, Berlioz's uncle arrives from Kiev at the apartment and is thrown out; shortly afterward, Sokov visits the same address and is received by Woland.
There was only one reason, the apartment. The apartment in Moscow!
Go back to Kiev immediately [...] pretend you are dead and do not dream of any apartment in Moscow, is that clear?
How much savings do you have?
In chapter 22, Korovyov explains to Margarita the fifth dimension that allows the apartment to fit an entire ballroom, and Woland receives her there.
I greet you, madam queen, and ask you to forgive my at-home attire.
In chapter 27, an armed squad storms the apartment, finds only Behemoth, and the cat sets it on fire with the primus stove's gasoline.
I'm not doing anything, I'm not bothering anyone, I'm fixing the primus stove [...] and I still consider it my duty to warn you that the cat is an ancient and sacred animal
What It Means
In chapters 6 to 12, apartment 50 works as a sinkhole, and in chapters 13 to 18 the consequences fall on whoever came too close to it.
Those who approach the apartment for the sake of housing are punished: Nikanor Ivanovich, harassed by requests for Berlioz's rooms, is arrested after accepting the rent and the wad, and the uncle from Kiev, who came for the inheritance of the property, is thrown down the stairs. Woland's line about the housing problem names the motive that brings these people there, and the same motive reappears outside apartment 50 in Aloisy Mogarych, who denounces the Master to take his apartment.