Final answer:
The substance that acts as a shield to prevent moisture from evaporating from the skin's surface is sebum, produced by sebaceous glands.
Step-by-step explanation:
The primary function of sebum is to act as a shield that prevents moisture from evaporating from the skin surface. The sebaceous glands secrete sebum, which not only waterproofs the skin and hair but also helps to maintain the skin's homeostasis by preventing water loss. Other structures like the epidermis, which contains keratin and glycolipids, and melanin, which absorbs UV light, also play significant roles in the skin's overall function of protection.