The poem is an elegy for Abraham Lincoln, and the line expresses the profound loss and realization of Lincoln's importance after his assassination.
Here's the full stanza from the poem:
> O Captain! my Captain! our fearful trip is done,
> The ship has weather'd every rack, the prize we sought is won,
> The port is near, the bells I hear, the people all exulting,
> While follow eyes the steady keel, the vessel grim and daring:
> But O heart! heart! heart!
> O the bleeding drops of red,
> Where on the deck my Captain lies,
> Fallen cold and dead.
>
> O Captain! my Captain! rise up and hear the bells;
> Rise up—for you the flag is flung—for you the bugle trills;
> For you bouquets and ribbon'd wreaths—for you the shores a-crowding:
> For you the drums are rolling, and the people all are shouting.
> But O heart! heart! heart!
> O the bleeding drops of red,
> Where on the deck my Captain lies,
> Fallen cold and dead.
>
> You never knew the worth of him until he died.
The poem uses the metaphor of a ship and its captain to symbolize the nation and its leader, Lincoln. The line you quoted expresses the powerful emotion of grief and realization that comes with loss. It highlights how we often don't truly appreciate the value of someone until they are gone.