This poem is an incredibly moving lyrical poem that reveals the feelings and thoughts of the speaker that emerge from within him as he listens to a woman singing a song at dusk time. You have quoted the first stanza, which introduces the poem and gives us the basic situation that takes the speaker back down memory lane and causes him to think of an early childhood memory. The first line gives us the setting and the situation, as the speaker hears the "soft" singing of a woman at dusk. This song has the impact of taking the speaker "back down the vista of years" as he thinks back to an early childhood memory of when he was little and used to sit underneath the piano whilst his mother played with her "small poised feet." The way that the mother "smiles as she sings" helps create an image of domestic bliss, and of intimacy and calm.
The first stanza helps to introduce the theme of the poem, which celebrates the power of music and also links it to memory, showing how a piece of music can transport us back through the years to happier times and states of innocence. It can also be said therefore to be a nostalgic poem that celebrates the innocence of childhood in the face of the complexities of adult life.