When you flip a class, you take the course content that would normally be presented in the form of a lecture and make it available to students online, often in the form of a video. So instead of students sitting passively during class time, they can take a more active role in group discussions or practical problem-solving activities. While this pedagogical idea is not really new, the ability to easily monitor how well students understand course material and provide them with immediate feedback is an added benefit of flipping and one of the reasons why this method has become so popular.
The Centre for Teaching Excellence at the University of Waterloo has published a superb web page with information about how to plan and implement the Flipped class method. Named in the article are several reasons why teachers should consider implementing the teaching method, important considerations to think about when planning a flipped class and some of the challenges to be expected.