The poem "A German Requiem" is full of contrasts, especially in the first and last stanzas with their repetitive structure and countless sentences of the form "It is not X. It is Y." In these particular lines, we're looking at a contrast between aloneness and togetherness: between sitting in your own house forgetting and taking part in a ceremony of forgetting together with others. Forgetting is the key concept in this poem. I think the following lines from the start of the third stanza are especially key to understanding what the poem is all about, because of their directness and transparency:
How comforting it is, once or twice a year,
To get together and forget the old times.
The ceremony being described - the journey in the "Widow's Shuttle", the gathering of "boiled shirts" in a cemetery to hear speeches, the short time spent tending the graves before going home - is all part of the so-called "ritual [of] oblivion" described in the first stanza. Let's look again at those lines:
It is what you have forgotten, what you must forget.
What you must go on forgetting all your life.
And with any luck oblivion should discover a ritual.
You will find out that you are not alone in the enterprise.
Yesterday the very furniture seemed to reproach you.
Today you take your place in the Widow's Shuttle.
This passage introduces the description of said ritual: waiting for the bus, dressed almost like a bride, gathering at the graveside, and so on. It's in these lines that the poem transitions from talking about forgetting to talking about communal forgetting. This is the contrast which the last two lines express: when the widow sits at home alone and ignores the past, the very furniture seems to reproach her for her forgetting; but when she takes her place together with the other widows, she receives validation and solidarity as they all sit and forget the past together.