The poem opens with a rhyming couplet that establishes a reflective, yet unsettling, tone:
I remember rooms that have had their part
In the steady slowing down of the heart.
"I remember" might sound like a nostalgic opening, but the second line hints that the speaker won't necessarily be sharing pleasant memories. The speaker is recalling rooms—not homes or other places associated with belonging—and the rooms they recall have played a role "In the steady slowing down of the heart." This phrase appears to be a metaphor for a gradual loss of energy, passion, love, etc. The speaker is remembering living spaces that seemed to age them, not places that stirred their heart.
It's unclear how or why these rooms sapped the speaker's passion, and that ambiguity will extend throughout the poem. The poem never describes any of the speaker's rooms in depth either, so it's impossible to know whether they were large or small, beautiful or ugly, etc. Since the details of each setting barely matter to the speaker, it's possible that these rooms reflect the speaker's emotional state more than they affect it. In other words, the speaker may be projecting their despair or exhaustion onto any and all rooms around them.
These first two lines establish the rough, four-beat accentual meter that will run throughout lines 1-7 of the poem. (Things get a little weirder starting in line 8.) Basically, each line contains four strong stresses, but the pattern/arrangement of those stresses varies a lot from line to line:
I remember rooms that have had their part
In the steady slowing down of the heart.
For example, there are two syllables ("that have") in between the second and third strong stresses in line 1, but only one syllable ("-ing") between the second and third strong stresses in line 2. The difference in the metrical rhythm helps convey the altered rhythm of the speaker's heart.
There's also a lot of alliteration in these two lines: "remember"/"rooms"; "have had"; "steady slowing." Both of the alliterative syllables in "steady slowing" are also metrically stressed syllables, so the alliteration heightens that stress—and adds to the sense that this second line is moving "slow[er]" than the first.