The poem starts off with two lofty, vague phrases that might seem confusing at first, as the speaker refers to "great pain" and the "formal feeling" that follows such pain. The first of these phrases simply refers to intense shock or trauma, but the speaker never bothers to specify the nature of the original “great pain” any more than that.
Perhaps this "pain" refers to heartbreak or grief over the loss of a loved one; given the morbid references in the following lines, death certainly seems to be on the speaker's mind!
But the speaker never clarifies what, exactly, this pain is because that's not the point here. The speaker doesn't seem interested in that pain itself, because the poem isn't focused on what people experience during a trauma. Instead, it's looking at the "formal feeling" that comes after said trauma.
"Formal" is the opposite of "casual"; it connotes the idea of ceremony or tradition, of stiffness, seriousness, and coldness. Basically, then, the speaker is saying that after great trauma comes a feeling of emotional numbness or paralysis; people seem suddenly to close themselves off or shut down.
The speaker personifies a sufferer's “Nerves” in order to illustrate this feeling further, saying that these nerves "sit ceremonious." Think about how you'd sit during a formal ceremony of some sort, like a wedding or graduation: quietly, a bit rigidly, immobile. And while the mention of ceremony here might briefly bring to mind images of a fancy, thrilling event or black-tie gathering, any positive connotations are quickly squashed by the comparison of these "Nerves" to "Tombs"—a simile that evokes a funeral. The sibilance of "sit ceremonious" recalls the quiet, hushed atmosphere of a funeral as well.
What actually does it mean to "sit" like a "Tomb"? Well, tombs are cold, inanimate objects, often dug into the earth. To "sit like Tombs" thus suggests being still and lifeless, forever in the cold and dark.
Imagine shocked, stricken people standing around an open grave. They aren’t weeping, or tearing their hair out in grief; instead, they're stiff and solemn, seemingly unable to express, or perhaps even, feel their anguish. This is the kind of "formal feeling" the speaker refers to.
The iambic pentameter in these lines adds to this sense of rigid formality. Iambic pentameter is perhaps the most common meter in traditional English poetry. It consists of five iambs, feet made up of one unstressed syllable followed by one stressed syllable. This gives iambic lines a steady, rhythmic feeling—almost like the beat of a funeral march (da-DUM da-DUM). Take line 1:
After | great pain, | a form- | al feel- | ing comes –
The meter isn't perfect here, but it's iambic enough to evoke that stiff beat. The trochee (stressed-unstressed) of "After" and spondee (stressed-stressed) of "great pain" suggest the force of such pain, the way it trips the speaker up before returning to the iambic plod.
The alliteration, especially in the first line, also emphasizes this rhythm. Notice how the /f/ sounds of "formal feeling" come at the beginning of a stressed syllable and draw readers' attention to this important phrase.