Do you mean Double Production? If so, it refers to doubling the rate of resource collection.... e.g., a resource that produces 2 food, 1 shield, and 1 trade would produce 4f, 2s, and 2t. I don't use it; the game balances better with standard settings, but some people like it in MP.
Or do you mean using gold to buy shields in the production box? If so, you just click "buy" and gold is deducted from your account, and shields are added to finish the item being constructed.