The speaker personifies the trees outside her window, talking about them as if they're gamblers playing poker. This makes the poem's setting feel alive, as if the speaker's surrounding world is full of life and activity.
However, this bustling sense of life profoundly changes in line 4 ("on a [...] yesterday"), when the speaker suddenly says:
You died yesterday.
This blunt statement juxtaposes the previous imagery of trees rustling in the wind and playing poker, changing the poem's tone from playful to somber.
And yet, the first three lines ("The big [...] ace high") do hint at this somber feeling. After all, the image of trees scratching against each other and shedding their leaves isn't necessarily all that joyful to begin with. In fact, this imagery brings to mind the passage of time, since leaves "turning" and dropping is symbolic of autumn. The fact that these leaves "float away" on the wind also adds a feeling of impermanence, as the speaker implies that nothing lasts forever. Before the speaker even says "You died yesterday," then, the poem subtly acknowledges that life is full of loss and change.
The speaker doesn't say who the "you" is, but the poem's title makes it clear that the speaker addresses her former teacher, who has just died. The caesura in line 4 highlights the sudden shift in focus, as the speaker goes from talking about the trees to acknowledging the teacher's death:
on a breeze. || You died yesterday.
Not only does the speaker pause before saying this, but she also end-stops the line so that the phrase "You died yesterday" feels like a very matter-of-fact statement. This emphasizes just how abrupt it can feel when loved ones die, as if death comes out of nowhere to claim them.