It is used to emphasize the truthfulness or intensity of something, much like the English words "really," "truly," "indeed," or "actually."
Here are some examples:
* Chungmal yeppeoyo (정말 예뻐요): Really beautiful.
* Chungmal baewo (정말 배워): Truly learned.
* Chungmal gamsahaeyo (정말 감사해요): Really thank you.
* Chungmal jal meokkesseumnida (정말 잘 먹겠습니다): Truly I will eat well.