A man has one small truth, and he clings to it the way you might cling to a night-light. It's simple and it comforts him. Then someone with "wider eyes" shows him a larger picture, full of truths piled on truths, and his settled world tips sideways. The word "turvy" is a playful, slightly comic touch, and the rhyme of "straight" with "curvy" shows how his tidy certainty has gone wobbly.
The ending is where the poem turns sharp. Rather than wrestle with the bigger picture, he cuts his truth in half because that's "easier to hold." One half he throws away. The other he keeps "till he sold," which suggests he eventually trades it for profit or convenience. The tone is light, almost nursery-rhyme-like, but the point stings: when truth gets complicated, people shrink it, discard it, or cash it in. The mood is wry and a little rueful, and the speaker seems to watch the man's compromise with quiet disappointment.