The horses are pulling the metaphorical carriage of the speaker's soul. With their heads pointed towards eternity, they are headed in the direction of the afterlife. In the last stanza, the speaker explains that it's been hundreds of years since he/she has passed away, yet each century feels shorter than the moment he/she realized mortal life had ended. Time is stretched and manipulated by perception, and for this particular person, the actual moment of death seems longer than the eternity the soul will exist.
Because I could not stop for Death,
He kindly stopped for me;
The carriage held but just ourselves
We slowly drove, he knew no haste,
And I had put away
My labor, and my leisure too,
For his civility.
We passed the school, where children strove
At recess, in the ring;
We passed the fields of gazing grain,
We passed the setting sun.
Or rather, he passed us;
The dews grew quivering and chill,
For only gossamer my gown,
My tippet only tulle.
We paused before a house that seemed
A swelling of the ground;
The roof was scarcely visible,
The cornice but a mound.
Since then 'tis centuries, and yet each
Feels shorter than the day
I first surmised the horses' heads
Were toward eternity.
Well, what is going on here? Or, maybe it's better to ask what has already happened. The speaker of the poem is already dead. Long dead. Time and space are contracted in this poem. Distorted.
The poem begins with the speaker and death in a carriage seemingly flying along with "immortality." The poem ends again with a reference to the carriage with horses leading it headlong into eternity. That's death. So we have a poem penned by a peson who is long dead. A weird perspective, but that's Emily Dickinson for you.