These two PPFs compare what Brazil and the U.S. can each produce per acre of land when choosing between Wheat and Sugar Cane. The shapes reveal fundamentally different production capabilities: Brazil's PPF is tall and narrow, so it can produce a lot of sugar cane but relatively little wheat per acre. The U.S. PPF is wide and short, so it excels at wheat but produces less sugar cane per acre.
A country has a comparative advantage in the good for which its opportunity cost is lower. Brazil gives up relatively little wheat to produce an additional unit of sugar cane, while the U.S. gives up relatively little sugar cane to produce more wheat. Even if one country were more productive at both goods (absolute advantage), trade based on comparative advantage still makes both countries better off because each specializes where its relative sacrifice is smallest.
Point A on each graph marks the maximum wheat output (if no sugar cane is produced), and point B marks the maximum sugar cane output (if no wheat is produced). The opportunity cost of one good in terms of the other is the ratio of these intercepts, and the country with the lower ratio for a given good holds the comparative advantage in it.