Dickinson frames "Because I could not stop for Death--" in an extended metaphor: she is being gently conveyed to her grave in a carriage in which Death is the driver:
We slowly drove--He knew no haste/And I had put away/My labor and my leisure too,/For his Civility--
These lines both establish the overall metaphor for the journey to the grave and employ two different figures of speech--death becomes personified as a carriage driver, and Dickinson employs the rhetorical device of litotes--understatement--when she describes herself not as having died but having "put away" the pursuits of a living person.
In the third stanza, which continues the metaphor of death as a journey, Dickinson observes a school in which children are playing, fields full of crops, and, last, "the Setting Sun," another metaphor for the end of one's life. With her penchant for startling images, however, Dickinson manages to turn this very conventional metaphor into something fresh: she and Death are not passing the setting sun but rather the sun is passing them, a reminder that, while life has stopped for Death and his companion, life goes on without them.
When they reach "a House that seemed/A Swelling of the Ground--," it is clear that the "House," with its "scarcely visible" roof, is actually the grave to which the two have been driving, a great example of periphrasis--circumlocution--to soften the harsh reality of the grave.
On the whole, Dickinson's use of metaphor and figures of speech in this poem contributes to the sense of revery and calm acceptance of death one feels while reading this poem. The harsh reality of death is entirely absent from this version of one's death, which is caste in terms of acceptance, peace, emotional objectivity, and the intellectual honesty that allows one to understand that life goes on.