Two sets of marks in the snow are set side by side here, and the poem lets the contrast do most of the work. The first is made by accident: a twig blown along by the wind leaves strange, shifting prints, none of them matching. It's random, a kind of nonsense writing that nobody meant.
The fox is different. She has walked over a mile, but on her own earlier footprints, so her trail has collapsed into a single blurred line with toes pointing both ways. You can read it as a clue to her habits: she goes out and comes back by the same path, perhaps to hide her trail or to save effort. The poem doesn't say why.
The speaker seems calm and quietly delighted, more observer than commentator. The mood is hushed and curious, the feeling of stopping to study the ground on a late-winter walk and noticing that even small marks can tell a story.