The scarcity of resources and trade-off in production levels of Products A and B relates to the fundamental economic concept of opportunity cost, which is defined as the cost of any activity in terms of the value of the next best alternative. In this case, opportunity cost is measured as the number of units of one product sacrificed to produce more units of the other product. The slope of the production possibilities frontier measures marginal opportunity cost: the number of units of B which must be forgone to produce one additional unit of A.
As in the plot below, PPFs usually have a concave shape which bows outward to illustrate the idea of rising opportunity costs. When the economy is producing few units of A and many units of B, and the frontier is relatively flat, the opportunity cost of an additional unit of A is low. But, when the economy is producing many units of A and few units of B, and the frontier is relatively steep, the opportunity cost of one additional unit of A is much higher.
Although it is not as common, a PPF can also be depicted as a convex shape which bows inward to illustrate falling opportunity costs (due to economies of scale), or a straight-line to illustrate constant opportunity costs (as is likely when resources are not specialized and can act as perfect substitutes for each other).