An economy can produce either guns or butter with all available resources.
Suppose it currently produces all butter and it chooses to produce some guns, thereby giving up some production of butter. As a result, the first 200 units of guns are produced at the cost of only 50 units of butter. Not surprisingly, the next 200 units of guns are produced at the cost of 100 units of butter and the last 200 units of guns at the cost of 300 units of butter. What does this describe?
The law of increasing opportunity costs