Scrum is an Agile type of project methodology. Agile means that both the teams and the requirements are flexible. The project is divided into iterations (repetition of some of the same processes within a project) of two to a maximum of four weeks. These are called sprints. Each sprint delivers a working piece/product, rather than splitting up the process in different phases with unfinished parts. Because of the short iterations, the customer has a much better insight into the project’s progress and the business and development team stay better aligned.
Agile and Scrum are the preferred way of working for bigger and more complex projects (Drupal).