A person sits alone in a shed and repeats the words an echo sends back. It sounds like a small, odd game, but the poem turns it into something quieter and stranger. The echo is described as "clear as the sea," and it arrives late at night in a still garden, so the voice seems to come from somewhere far bigger than the little space around the speaker.
The last stanza makes the shift. The echo lets the speaker hear "children in the water" and then "in myself." The sound brings up the noise of play, perhaps something remembered, perhaps something that lives on inside. The poem doesn't say which, and that openness is part of its charm.
The mood is hushed, private and a little wistful. The speaker isn't lonely in a sad way. Instead, being alone makes it possible to listen closely, and the repeated words become a way of hearing both the world outside and an earlier, more playful self within.