From the human history people have worked together for goods , for example if there is a pizza shop there would be people with different work , one with baking the pizza , one with adding sauces to it and one with packing it , one with making cheese by cows , one with making the logos , boxes and microwaves and etcetera , humans have followed this sequence through out centuries , from the stone age till now.

This is a production possibility frontier , this explains the condition of 2 products , for example if India produced 500 planes with there labor a day and also can produce 1000 tons of shoes a day , they cant do both , India can only have limited amount of resources thus they could only spend it on one quantity product.

If the production of planes and shoes in tons are inside the perfect production line then they are inefficient , meaning not that great of a product , if they are out of the perfect production line then it is not possible as it goes above the goods and service limit , thus it is considered impossible

China produces 100 quantity of planes everyday and also produces 800 tons of shoes , but US produces 500 planes a day with 1000 tons of shoes thus US is more efficient and greater in economy of planes and shoes than China is in planes and shoes.

US can produce 1 plane for the loss of 2 tons of shoes while China can produce 1 plane in the loss of 8 tons of shoes , US can trade 1 plane to China for 4 tons of shoes , it helps China and US