Three short lines give us a small, quiet scene: a girl alone in a bathtub while the light above her flickers on and off. The surprise comes in the last line, where she "flickers back." She doesn't just sit there while the light acts on her. She answers it, as if she and the light were having a private conversation.
The poem never says what the speaker feels, and that's part of its pull. The tone is calm and observant, almost like someone watching through a doorway. But "flickers" is a word we normally use for light, not people, so it hints that the girl is fragile or unsteady, maybe fading in and out of her own mood or presence. Judging by the wording, she could be lonely, tired, or simply lost in thought.
The mood is hushed and a little eerie, but also strangely gentle. It leaves you with a feeling of closeness and uncertainty at once.