A feral cat returns after a winter away, and the speaker simply notices it. There is no explanation of where it has been, only the phrase "wherever it goes for the winter," which admits that the cat's life is mostly hidden from us. That small admission gives the cat a quiet independence. It belongs to no one, and it comes and goes on its own terms.
The scene is still and focused. The cat crouches on a fallen limb, its eyes fixed on the weeds. The last line, "gathered for the spring," is a little ambiguous. It could describe the weeds, which have started to gather and grow, or it could describe the cat itself, tensed and collected, ready to pounce. Either reading fits.
The speaker doesn't state a strong feeling, but the poem seems to carry a calm pleasure at seeing the cat again. The mood is watchful and patient, like the first signs that winter is over.