I think this means that if you want to communicate as closely as possible to a native speaker, you need to learn not only "what" to say, but also "how" to express your meaning. This skill will include authentic-sounding intonation, with matching gestures, facial expression and body language that will mimic a native speaker. Otherwise, you may have a wide and sophisticated vocabulary with a good mastery of grammatical structures, and use these appropriately according to context, but still sound very "foreign", if your delivery of the language is not mastered. So, you need to "inhabit" the language, much like an actor inhabits her roles.