The iced fish display functions like a retail fridge and is where you can store any fish you want to sell. Sims will come up to the display and browse for a while before deciding to purchase stuff from it. You can open it at any time to see what's in it's inventory.
Note: please be aware you cannot normally store fish in the retail fridge and you'll need my mod HERE to put fish in this object.
May contain spoilers!
These mods are required for this content to function properly in-game. Please read through all the information on the download pages so you understand what's included with that mod and how it works.