A person has a comparative advantage at producing something if he can produce it at lower cost than anyone else.
Having a comparative advantage is not the same as being the best at something. In fact, someone can be completely unskilled at doing something, yet still have a comparative advantage at doing it! How can that happen?
First, let's get some more vocabulary. Someone who is the best at doing something is said to have an absolute advantage. Michael Jordan has an absolute advantage at basketball. For all I know, Michael Jordan may also be the fastest typist in the world, giving him an absolute advantage at typing, too. Since he's better at typing than you, can't he type more cheaply than you? That is, if someone has an absolute advantage in something, doesn't he automatically have a comparative advantage in it?
The answer is no! If Jordan takes time out from shooting hoops to do all his own typing, he sacrifices the large income he earns from entertaining fans of basketball. If, instead, his secretary does the typing, the secretary gives up an alternative secretarial job—or perhaps a much lower salary playing basketball. That is, the secretary is the lower-cost typist. The secretary, not Michael Jordan, has the comparative advantage at typing! The trick to understanding comparative advantage is in the phrase "lower cost." What it costs someone to produce something is the opportunity cost—the value of what is given up. Someone may have an absolute advantage at producing every single thing, but he has a comparative advantage at many fewer things, and probably only one or two things. (In Jordan's case, both basketball and also as an endorser of Nike.)
Amazingly, everyone always has a comparative advantage at something. Let's look at another example. Suppose you and your roommate want to clean the house and cook a magnificent Chicken Kiev dinner for your friends one night. The easy case is when you are each better at one activity. If you are an accomplished chef, while your roommate doesn't know the range from the oven; and if after you vacuum the carpet the dust bunnies have shifted from under the sofa to under the coffee table, while your roommate can vacuum, dust, and polish the silverware faster than you can unwrap the vacuum-cleaner cord, then you and your roommate will each be better off if you cook and your roommate cleans. It's easy to see that you each have a comparative advantage in one activity because you each have an absolute advantage in one activity.
But what if your roommate is a veritable Martha Stewart, able to cook and clean faster and better than you? How can you earn your keep toward this joint dinner? The answer is to look not at her absolute advantage, but at your opportunity costs. If her ability to cook is much greater than yours but her ability to clean is only a little better than yours, then you will both be better off if she cooks while you clean. That is, if you are the less expensive cleaner, you should clean. Even though she has an absolute advantage at everything, you still each have different comparative advantages.