A dependency is a way to mark one task dependent on another before it can be completed. It’s saying “Task B cannot be worked on until Task A is first completed”. Dependencies in Stuff allow you to hide away tasks which are not currently relevant (because they cannot currently be worked on), then make them visible once they are available to be worked.
Dependencies work a little differently on Shared Lists than throughout the rest of Stuff. Whereas tasks in Stuff can be dependent on any other task in any other list or space, tasks in Shared Lists can only be dependent on other tasks within that same list.
If a task in a shared list has a dependency on another task in that list, then is moved out of the list, it will remove its dependency on that other task.