The title points to Descartes, the philosopher who worried about how we can tell being awake from dreaming. Bonta answers in four short lines, and he doesn't argue the point. He offers an image instead.
Having a falcon at your door means something real and present: a living bird, with weight and sharp feet, that you could actually handle. Having the moon at your door is something else. It is beautiful, but impossible, the sort of thing that only happens in a dream. So the speaker suggests that waking life is solid and specific, while dreaming is vast and glowing but out of reach.
The mood is quiet and a little wondering. The speaker doesn't sound anxious like a philosopher doubting reality. He sounds more like someone gently comparing two kinds of experience, and perhaps feeling the pull of both. The moon at the door is a lovely thing to imagine, even if it isn't true.