iwhat is goodbye in Korean?

There are two expresstions for goodbye in Korean.

One is "안녕히 가세요.", the other is "안녕히 계세요."

안녕히 is an adverb which means "in peace, comfortablely"

가세요 = 가다(to go) + -세요(imperative, honorific)
계세요 = 계시다(to stay) + -어요(imperative)

So, 안녕히 가세요 means "Please go comfortably, safely, peacefully..." and "안녕히 계세요" means "please stay comfortably, safely, peacefully."