In Aboard a Task can have a parent and child like relationship where the child task will be blocked until the parent task is completed.
Go to Program or Journey
Add or Edit a task
Check Trigger this task after another task
Select a parent task

How it looks in Timeline view:

How it looks from Tasks:

When a child task is deleted, the relation with its parent will be severed.
When a parent task is deleted, there will be options to choose which child tasks to be deleted and/or orphaned.
