Let’s think about these in terms of individuals instead of nations and choices we all understand... like trying to decide whether to become a fry cook or a heart surgeon. (That’s a debate we have internally just about every single day.)
Let’s say you’re a wonderful fry cook and a talented heart surgeon. Your neighbor is a pretty good fry cook and (since he is near-sighted and has a severe hand tremor) an absolute butcher as a surgeon.
You have an absolute advantage over your neighbor as a fry cook. He just can’t hang with you around a pan of sizzling lard. And you also possess an absolute advantage over him as a surgeon—your patients actually survive their surgeries occasionally.
Which career should you pursue? To become a fry cook, you must sacrifice your far more lucrative work as a heart surgeon; your opportunity cost is very high. On the other hand, your moderately skilled fry-cooking neighbor has a relatively low opportunity cost—to pursue a career as a fry cook he only has to sacrifice his malpractice-suit-waiting-to-happen career as a surgeon. Therefore your neighbor has a comparative advantage in fry cooking... even though you’re objectively a better fry cook than he is.
Do you know what you want to be when you grow up? Choosing a career is one of the most important choices you’ll make in your entire life. If you don’t want to make the wrong choice, you’d better think about comparative advantage—both your own as an individual and your country’s in the international market.
If you’re living in the United States, it really doesn’t matter if you’re the world’s most talented stitcher of men’s underpants. Other nations where labor is much cheaper (hey there, Indonesia!) have such a strong comparative advantage over the high-wage United States in the underwear assembly industry that you’ll never be able to find and keep a job in that field.
Stupid. We are talking about labor arbitrage across international borders. Competition between Americans is good. Competition with the third world - bad. Very bad. Just shows deep down you are a stateless creature. Pathetic.