It means the action itself to which it is attached. Usually, It is used to provide us information about the circumstance of an action. For example, "말한 바가 훌륭했다." can be translated "What you said were great." or "선생님이 들어온 바, 시험이 시작한다." "The teacher comes in the class so the test is starting." "~는 바" usually is used in literary style.