Here's the relevant stanza:
> 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,
> For His Civility –
>
> We passed the Fields of Gazing Grain –
> We passed the Setting Sun –
> We passed the Woods – and reached the Town –
> And then the Brooch of Evening drew
> Upon the West a Ring
> The Stars began to burn
> Like distant Men that smiled
> Then I knew that I was Dead
The "Brooch of Evening" is a metaphor for the setting sun, which acts like a piece of jewelry adorning the sky. This imagery emphasizes the beauty and inevitability of death.