The narrator in "Because I could not stop for Death" faces a problem many people have: she is too busy to die. Nevertheless, Death (personified) stops by to pick her up, and thus they begin their journey into Eternity.
As they travel, the narrator passes by several metaphorical representations of her life in review. She passes a school, symbolic of her own childhood; she passes a field of gazing grain, symbolic of her midlife; and she passes the setting sun, symbolic of her own death. And then they pass this House, which is the narrator's own grave. Other hints that this is her grave follow in this stanza. First, she notes that it is a swelling of the ground. Unlike today's graves which are dug with large tractors and machines and then leveled off even with the surrounding ground, the graves of this time period, especially when freshly dug/covered, would have created just this image—a little ground swell, all puffed up. The cornice mentioned (which in architecture is typically a decorative crown at the top of a building) is likely her own tombstone, now crowning her grave. She notes all of these things, which many might consider shocking or morbid, in a very emotionless and matter-of-fact tone.
Although the narrator reflects that this journey into Eternity happened centuries prior, she notes that it feels like it's been less than a day. In this poem, the narrator doesn't seem to feel particularly emotional about either dying or death and instead simply takes readers along for a glimpse into the beginnings of her afterlife.