In this poem, there is a girl who is preparing herself for her day. The last three lines that you are asking about illustrate her looking in the mirror of a pond, as illustrated in these lines:
Her face appears in the mirror,
a reflection in a winter pond,
rising to meet itself.
She is looking at her reflection and as she's about to "paint herself" she wipes the reflection with her sleeve. I get the feeling that she is not happy with her situation in life. I think perhaps she's a geisha or something similar. The morning at the pond, or when she's looking at her reflection is the only time that she sees the real person inside of her.
The eyes narrow
in a moment of self-scrutiny.
The mouth parts
as if desiring to disturb
the placid plum face;
break the symmetry of silence.
But the berry-stained lips,
stenciled into the mask of beauty,
do not speak.
The fact that she can't speak, that the poet calls her face a "symmetry of silence" tells us that she leads a "secret" life that she doesn't like to admit or talk about.
In the last three lines, Song writes:
touch in the middle of the lake
and drift apart.
In this case, the one flower has to go to do what she has to do, while the other waits in the pond for her to come back tomorrow.