The narrator of this poem is listening to a woman singing while she plays the piano, except that he's not really listening to the music happening around him. Instead, he's listening to the music in his memory, which is taking him back to his childhood and the time he enjoyed sitting under the piano while his mother sang and played. In the first stanza, he describes himself as
A child sitting under the piano, in the boom of the tingling strings
And pressing the small, poised feet of a mother who smiles as she sings.
In the second stanza, the narrator confesses that the song being sung in the present is transporting him, in sorrowful memory since his mother is no longer living, "back...To the old Sunday evenings at home, with winter outside And hymns in the cosy parlour."
The third and final stanza is the narrator's giving himself over to his mourning for days gone by. The "great black piano appassionato" and melodic singing of the woman in the room with him is unnoticed and unappreciated. The narrator is overcome with memories of his childhood and he can only "weep like a child for the past."
A brief paraphrase of the poem could simply read: I hear the singing and the piano music, but all it does is remind me of the times I spent sitting under the piano while my mother played and sang. How I miss those wonderful nights of warmth and security and loving music at home. The singer's beautiful voice and wonderful piano playing is wasted - I am lost in my memories. How sad that I can never return to that time.