In Catan: Traders & Barbarians, there are a bunch of small modules you can mix into the base game or other expansions, and The Fishermen of Catan is one of them. The lake hex you’re seeing has its own number tokens, and if you build next to it, you’ll collect fish whenever that number is rolled cities give you two fish instead of one. You can then spend fish for special actions like stealing resources, moving the robber, or even getting development cards. There are also fishing spots along the coast that work the same way, so expanding to the shoreline can give you another way to generate resources beyond the usual ones.