This project management model is also called Triple Constraint or the Iron Triangle. It helps to illustrate that no project-related constraint is independent of the others. If one constraint is being changed, for whatever reason, it naturally impacts the other constraints and leads to changes in them as well.
As the constraints are usually competing with one another, one side of the triangle cannot be changed without automatically affecting the other two sides. The project manager has to assess how a tight budget might affect the other constraints. Facing a cost bottleneck might mean that it will take longer to complete the project or it might require a reduction in scope.