Because I could not stop for Death –
He kindly stopped for me –
The Carriage held but just Ourselves –
And Immortality
We slowly drove – He knew no haste
And I had put away
My labor and my leisure too
The ‘Carriage’ may be a hearse, which the dead speaker shares with allegorized Death, but also with ‘Immortality’. Dickinson intimates that death is a necessary part of the passage to the next life. There are also romantic overtones, i.e., the speaker is on a date with Death. Or the carriage may simply be a fantasy, the vehicle of an imagined journey that the speaker is taking into the realm of death. 
 The speaker and Death are about to travel to the grave, depicted in stanza five as a ‘House’. This journey might be a funeral procession or a dramatization of the “life flashing before your eyes” scenario. 
 Dickinson may be hinting here at  poetic  ‘immortality’; i.e., she is graced with the presence of immortality because her art will live on. (An old trope that’s alive and well today: see Jay Z’s  “Threat.” ) Alternatively, this could simply be Dickinson’s original, highly unconventional vision of a Judeo-Christian afterlife.
Because I Could Not Stop for Death