In "Piano," the narrator finds himself in a setting where a woman is singing, accompanied by piano. This sound stirs something within his soul, and he is suddenly transported back in time to his childhood.
Many find this true about the power of music—its ability to trigger memories and emotions. This is certainly true for the narrator, and he not only remembers sitting under the piano at home but also remembers his own mother playing for him. He remembers her smile and how the music also stirred something within her, a joy that made her "smile as she [sang]." The narrator felt both emotionally and physically connected to his mother as she played; he was literally pressing her feet as she created this thing of beauty for him.
In the present, the narrator suddenly longs to be back in this scene, to feel that warmth and the serene joy the music created on these evenings at home. He notes that the music kept "winter outside," metaphorically creating a barrier between the cold world outside and the warmth within. He also connects the music to a deep spirituality, noting that he remembers the music most often on "Sunday evenings" and that his mother played "hymns" in their "cosy parlour." His mother was a master of song and of creating warmth on these cold evenings, and his soul now longs to weep for what no longer exists.
He says that the singer he listens to now, in the present, has no hope of stirring his soul in the same way, so it is "vain for [her] to burst into clamour." The real glamour has passed; it is found in the days of his childhood and in the simple memories that his mother created, borne out of genuine love for him and their family. As he recalls once more those evenings, he notes that his "manhood is cast / Down in the flood of remembrance." He gives in to the emotions stirred initially by this piano music in his present, and he weeps for the past and for the beauty music created for him as a child.