Three short lines, and almost nothing happens. Ice sits on water, and then the poem steps back to say it is an evening at home. That is the whole scene, and its smallness is the point.
The first two lines give us a plain image: ice resting on liquid, solid and fluid side by side. The word "sitting" is a small surprise. We usually say ice floats, but "sitting" makes it sound like a person settled in a chair, and that quietly links the ice to the final line. The evening at home feels the same way: settled, unhurried, going nowhere.
The speaker never says how they feel, but the mood is calm and a little cool. It could be a glass of water with ice in it, or something colder outside; the poem doesn't say. Judging by the wording, the speaker seems content simply to notice. The poem asks us to slow down and enjoy a quiet, ordinary moment, the way ice slowly sits and melts.