The Law Of Increasing Opportunity Cost says that one must give up larger and larger quantities of something, in order to get something else.
Look at an example.
Take cotton. A nation decides it needs more cotton, ceteris paribus.
This means that land that was in the past not farmed will be now used to grow cotton.
At any rate, the cotton grown per acre will fall.
More and more land must be used to grow marginally decreasing amounts of cotton.