Those who failed the exam take a makeup exam.

这次考试不及格的人必须参加补考。

So the first one is really used like an adjective: a makeup exam.

所以第一个被用作形容词：补考。

When we put it together, we have makeup. You can do the makeup exam, means, you do it again.

把它们放到一块时，我们就有了“makeup”。你可以补考，意思是再考一次。