Three short lines, and almost nothing happens: a dog is wearing a collar that has twisted out of place. What matters is the small observation the speaker makes about it. The dog doesn't mind. He isn't fussing, scratching, or trying to fix it. He just carries on being himself.
The speaker's feeling is never stated, but the tone suggests quiet affection, perhaps with a touch of envy. Notice that the speaker is the one who sees the twisted collar, which hints that a person might care about a detail like that far more than the dog does. There's a gentle contrast between human fussiness and animal ease.
The mood is calm, warm, and a little amused. Because the poem is so brief, it feels like a passing thought caught mid-moment. It invites you to wonder what else we worry about that doesn't really matter, and whether we might learn something from a dog who lets it go.