Three short lines, and almost nothing happens: someone sits on the floor and cries. That's the whole poem, and the smallness is the point. We aren't told why the speaker is upset, so we can't explain the feeling away or judge whether it's a good enough reason. We just get the fact of it.
Sitting on the floor suggests the speaker has run out of strength or has stopped caring about composure. They haven't made it to a chair or a bed. They've simply dropped to the lowest place available. The line breaks add to this. "I sat on" hangs in the air before the floor arrives, and the final word, "cried," lands alone, like a sob that finally breaks through.
The mood is bare and quiet, and a little exposed. Because the poem withholds the cause, readers can bring their own sadness to it, which makes it feel oddly close and personal despite its brevity.