Each country has 100 worker hours. Saudi Arabia needs one hour for a barrel of oil and four for a bushel of corn, so it can make 100 barrels or 25 bushels. The United States needs two hours for a barrel and one for a bushel, so it can make 50 barrels or 100 bushels. Those extremes are points A and B on each frontier, and the straight line between them shows every mix in between.
The slope of each line is the opportunity cost of oil in terms of corn. Saudi Arabia gives up a quarter of a bushel for a barrel; the United States gives up two bushels. Saudi Arabia therefore gives up the least to make oil, which is its comparative advantage, and the United States gives up the least to make corn. The sliders move each country's chosen point along its own frontier, and the panel keeps the running total of world output.