In just a few lines, this short piece sets two images side by side. First we see a couple of ducks, calmly eating while a cold wind blows around them. Then the speaker turns to themselves: "this ache is my Valentine."
The contrast does most of the work. The ducks are a pair, and they seem comfortable even in harsh weather. The speaker, by implication, is on their own, and the only thing they have to mark Valentine's Day is an ache. The poem doesn't say what causes it. It might be loneliness, loss, or longing, and that openness lets readers fill in their own version.
The mood is quiet and wistful, with a dry, self-aware humor in calling pain a Valentine. The title's word "idyll" usually suggests a peaceful country scene, so it adds a note of irony. The scene looks gentle, but the speaker feels the cold more than the ducks do.