This poem has arguably one of the most arresting and intriguing openings in English language poetry. A first-person speaker, later revealed to be a woman through the description of her clothing in stanza four, encounters "Death." This Death is a personified figure, arriving to pick the woman up as though he is running some kind of 19th century taxi service.
But, of course, this ride comes about not because the woman wants it to, but because it is her time to die. She "could not stop for Death"—and, indeed, not many people would choose to do so—and so it is Death's responsibility to stop for her. The description of Death as "kindly" in line 2 suggests a gentlemanly figure, one who is going about his duty while behaving respectfully to those he picks up. It's notable, though, that as with the rest of the poem Death remains silent here. Death remains a mysterious, shadowy figure, even if he isn't presented as something fearsome and terrifying.
The woman climbs into Death's carriage and makes another mysterious observation. Describing those present within the vehicle, she mentions that, other than herself and Death, "Immortality" is also present. This moment is highly ambiguous, perhaps deliberately so. It's not clear if, like Death, Immortality is a personified figure—and another silent type at that. Alternatively, immortality may just be present in a more abstract sense as part of the atmosphere of the carriage.
The poem's sounds help capture the nuances of this opening stanza. The consonance of /l/ sounds in "held" and "Ourselves" seems to cling to the mouth of the reader, evoking the way in which this is, put simply, not a voluntary journey—that is, despite Death's gentlemanly civility, he also forces the speaker to join him.