Because I could not stop for Death
Death, in the form of a gentleman suitor, stops to pick up the speaker and take her on a ride in his horse-drawn carriage.
They move along at a pretty relaxed pace and the speaker seems completely at ease with the gentleman. As they pass through the town, she sees children at play, fields of grain, and the setting sun. Pretty peaceful, right?
As dusk sets in our speaker gets a little chilly, as she is completely under-dressed – only wearing a thin silk shawl for a coat. She was unprepared for her impromptu date with Death when she got dressed that morning.
They stop at what will be her burial ground, marked with a small headstone.
In the final stanza, we find out the speaker's ride with Death took place centuries ago (so she's been dead for a long time). But it seems like just yesterday when she first got the feeling that horse heads (like those of the horses that drew the "death carriage") pointed toward "Eternity"; or, in other words, signaled the passage from life to death to an afterlife.