It means linking two tasks in order to provide an easier reference between related tasks. There are 3 kinds of links that can be defined. They are Related to, Duplicated by, Derived from.
Related to: – If a task related to the current task exists, then you can select Related to in order to indicate that the 2 task are related.
Duplicated by: – If the similar type of task is added before, then you can use Duplicated by in order to link the tasks
Derived from: – If the current task is the result of or derived from a previous task, then you can use Derived from to link the task.