Golden Shovels are used to search for collection items on the Big Land and on the Archipelago. 
Buy a Moneybox (Market >> tab with arrows), tap on it or on the icon with blue arrows in the bottom right corner, select the golden shovel icon and tap the objects or swipe through the field where you want to search.
You can search under trees, rocks and all kinds of Decorations. If you don't have any trees or rocks left, just buy some Decorations at the Market, place them on the Big Land and enjoy the treasure hunt!