The Agile Triangle is a paradigm created to communicate the three practice areas of Agile Software Development: Product Definition, Delivery Framework, and Engineering Practices.
This paradigm is needed because many software development organizations have the idea that Scrum is an all encompassing methodology or process. Many do not understand that Scrum is only part of the Agile picture (the Delivery Framework); and that Product Definition and Engineering Practices are necessary for agile software development.
The Scrum Guide even states that “Scrum is not a process, technique, or definitive method. Rather, it is a framework within which you can employ various processes and techniques.” So if you are not employing Product Definition and Engineering Practices, you are working with an empty framework that is missing processes and techniques.