In just three lines, the speaker takes stock of a modest piece of land, a half-acre, and finds it sufficient. The key word is "admit." It makes the land sound like a doorway or a host, something that can let the moon in, as if the moon were a guest arriving by invitation. A half-acre isn't much, yet it's enough to share space with the sky.
The title, "Bucolic," points to a peaceful, rural life, and the poem fits that with a quiet, contented tone. There's a gentle humor in it too: the speaker measures the property in acres, a practical, ownership-minded unit, and then sets it beside something no one can own. A quarter moon is only a slice of light, which suits the small scale of the place. The feeling is modest satisfaction, even gratitude. The speaker doesn't wish for more land. What matters is that this small patch stays open to the night sky and lets something larger in.