Selecting concepts and content is a study method. The explanation can be better understood by the example. For example if you are a student of history are studying something about Babylonian family system, it will always be easy for you if you keep the concepts highlighted which will make it easy when you are revising. For example when studying in the text you can highlight the concept that “Father was the central figure of Babylonian family”. Now revising will always be easy when you read the concept.
Content selecting is that when you have highlighted the concept of family, the content can be that what was the role of women in that society or how much power the father or male head of the family had over other members etc.