What does the bowed-out shape of the production possibilities curve illustrate?

The bowed-out shape of the production possibilities curve (PPC) illustrates the Law of Increasing Opportunity Costs, which states that as an economy produces more of one good, the opportunity cost (the amount of another good sacrificed) for each additional unit of that good increases. This occurs because resources are not perfectly adaptable to the production of both goods; to produce more of one, less efficient resources must be reallocated, making it progressively harder to produce more of the other good.