The poem deals with an encounter between the speaker and the personified figure of Death. Instead of being presented as a scary hooded figure like the Grim Reaper, Death is given to us as a charming gentleman riding a horse-drawn carriage. Death shows us his impeccably good manners by stopping and inviting the speaker to hop aboard his carriage: "Because I could not stop for Death/He Kindly stopped for me."
Death then proceeds to take the speaker on a journey towards eternity. In other words, it's likely that this is the speaker's last day on earth. On the way there, they pass a school where the children are gathered in a circle. It looks like they're playing a schoolyard game, but it's most probably the case that they're trying to work out their futures; the circle in this case could be seen to represent the journey of life and death.
That this journey is no ordinary one can be seen by the fact that the sun passes the carriage instead of the other way round. All of a sudden, the speaker feels cold as the light and the warmth of the sun start to fade. In case we didn't know it already the speaker is in the process of leaving this world for the next.
In the penultimate stanza, we're presented with a large mound of earth beneath which appears to be buried a house. We don't know whose house it was, but it's more than likely that it once belonged to the speaker herself. In any case, it now resembles a grave more than a dwelling-place, which is perfectly in keeping with the overall theme of the poem.
Although the speaker tells us that this all happened hundreds of years ago, we can't be entirely sure because, as someone who's dead, she's lost all track of time. Her supernatural existence makes it seem to her that these events happened in less than a single day.