The winning combination: A deep understanding of the content and/or skills you are teaching and knowledge of how to teach the content to your students in developmentally appropriate and engaging ways. Teachers who know their content can take complex ideas and subjects and teach them successfully to students at different ages. Content learning targets are easier to identify and coach students toward when they are clear and supported by teacher understanding of the material as well as how the learning targets fit into the structure of the discipline. Of course, no matter how well you know the material, if you cannot teach it to your students in a way that gets them to learn it and think further about it or apply it, then knowing the content is for naught!
You might want to start here:
See it in action: