Three short lines hold a quiet moment at the end of a day. The speaker hears crickets and calls their chirp "the last lost sound of this Saturday," as if the day is slipping away and the noise is the final thing left of it. Then the focus turns inward: a baby stirs in sleep, "inside." That last word suggests the baby is still unborn, so the speaker may be an expectant parent feeling a movement, though the poem never says so outright.
The feeling is gentle and a little hushed. There's a touch of wistfulness in the word "lost," as the day fades, but the baby's small movement brings warmth and anticipation. One thing ends while another is just beginning. The mood is calm, intimate, and attentive, with the speaker noticing tiny sounds and motions in the stillness of night. The poem also works by contrast: the world outside winds down, while new life quietly makes itself known.