This is a popular flannel pearl snap shirt that one can wear on a brisk day fishing the stream then to the local hangout to swap stories.
From the Simms website: “Burly and Montana-styled flannel that can handle plenty of gear and take you from the drift boat to your favorite restaurant. Features: Water‐resistant. Quick‐drying, wicking, anti‐odor, and UPF 50. 2 zippered chest pockets. 2 fly box‐compatible chest pockets.”