The carriage in "Because I could not stop for Death" symbolizes the journey from life into eternity.

The speaker in this poem believes that she is simply too busy to die when this carriage stops by to pick her up. The other passenger in the carriage (besides the speaker) is Death, and their eventual destination is the speaker's final afterlife, likely Heaven.

Along this journey, they pass several scenes which symbolize portions of the speaker's life. First, they pass the school where children play outside; this represents the speaker's childhood. The children also play in a ring formation, which symbolizes the cyclical nature of life and death. She then passes fields of "Gazing Grain," which represent her adult life. The grain isn't newly planted (which connotes young life), and it isn't quite ready to be harvested, either. The idea that the grain can gaze back at her perhaps symbolizes the inevitable sense of eventual death that settles in sometime in adulthood. Her midlife self had the foresight to realize this eventual journey would she would embark on with Death. They then pass the setting sun, which represents the speaker's final moments in life. Finally, they pass the speaker's own grave, represented in these lines:

We paused before a House that seemed
A Swelling of the Ground –
The Roof was scarcely visible –
The Cornice – in the Ground –

The "House" which swells out of the ground can be visualized as the mound of dirt which typically represented graves in this era. The "roof" which sits atop this "house" is the tombstone for the grave.