This first line gives us a lot of vital information about what's going on in the poem. If this poem were a real epitaph, as suggested by its title, this would be written on the old woman's gravestone. Yet, we're not reading this on a gravestone, so some background information is provided in this line.
This woman has a family tomb and is being buried by "they." If you've never seen a family tomb, picture a little building, or perhaps a little gated area, which contains the graves of all the members of a family, spanning generations.
"They" is probably the younger members of her family, maybe a son or daughter, with grandchildren gathering around. We can imagine that the sadness and weight of the death of this old woman, who has lived a long life, sits heavy on the shoulders of the crowd.
Or maybe there are only a few people watching her burial—we have no way to know that this woman ever had children, though we can assume that "they" include family members, or at least close friends, who enter her into the tomb.
and in the depths the dust
This line takes us down into the family tomb, with the woman who is being buried. The word "depths" lets us know that this tomb goes down underneath the ground, unlike some tombs, which have above-ground structures. There is dust down there, and at this point, we don't know what this dust is. It could be the kind of dust you'd expect in an underground vault, a fine layer covering the coffins of the dead, trapped far from the sun.
Of course, all this talk of tombs and dust also reminds us of the saying, "ashes to ashes, dust to dust," which is modified from a verse in Genesis, in the Bible.
So we won't make conclusions until we read the next line…
of what was once her husband
Aha! And here we find that this dust isn't normal dust. Instead, it's the dust of the remains of the husband's body. This means that he was probably buried a long time ago, and has been in the tomb, turning back to the dust which, according to the Bible, he came from.
Whether the significance of the dust is biblical or not, this line makes the reader think about the way our bodies naturally decay at the end of life. The old woman is being lowered into the grave, next to her late husband, yet it's only his dust that she is joining.
As you read this line, imagine, for a moment, that this woman has missed her husband since the day he died, years ago. Now, though she is not alive to know it, she's finally joining him. He, too, has been alone in the tomb without her, waiting for her in whatever afterlife may or may not be.
Note the arrangement of this line. We don't find out that this dust was once her husband until the very end, building our tension and expectations for when we finally get that moment of breakthrough.
This moment in the poem is sweet and sad—but there's no period at the end of the line. Our eye is already drawn to the next line because of its indentation, making it closer to the middle of the page.
In this line, we go from the natural process of death and decay to the supernatural, or at least to the wildly imaginative. We knew that we weren't done with the husband's dust at the end of the last line, and, conveniently, this next line is indented nearly to the middle of the page so that it catches our eye right away.
We know that this line is pretty momentous because of this indent. We also know because it's just one word, making it different from the rest of the lines in the poem. This one word, "trembled," suggests that the dust of the old woman's husband is somehow aware that she is being lowered down to be with him.
Yet we don't know what kind of tremble this is. Suspend your belief, and allow, for a minute, that the dust is actually capable of trembling. Is he trembling with joy at being joined with his wife? Or with sadness that she, too, has left the world of the living? Or is this merely disturbing his sleep?
You may have heard someone say "If so and so knew this was going on, she or he would be rolling over in their grave." Well, the wonderful thing about poetry is that wild things like rolling over in a grave can happen. When we think about the dust trembling, it opens our minds up to think about the possibilities of love and life after death.
Note that this line ends with a colon. This means that what follows is either a list, or an explanation—a definition of sorts.