Dependencies specify the relationships between tasks, milestones, and other items which can be presented on the timeline and can be displayed by the Gantt and Board type modules (Gantt and Board are the default module names used).By defining dependencies, you can not onlyautomatetask scheduling but also highlight relations between tasks without impacting the schedule. Keep in mind, that the items presented on the Gantt chart are system-wide and changing their dependencies might affect tasks in other Boxes or connected tools.
External links show dependencies between tasks within the scope of different Boxes. Such a dependency might constrain and therefore the task period mode is set to "locked":