Subtasks are smaller divisions of a task. When tasks are complex or involve a lot of people, it is useful to break them down into subtasks. For example, "Creating Promotional Material" may have the subtasks "Content Creation" and "Graphic Design".
There are two ways to create a subtask. You can put your cursor on the dotted line on the left side of the bar until a cursor arrow appears. Click on the task, drag it and bump it against the one you want to be the parent task.
Another option is to display the edit page of the task you want to set as a subtask. Select the one that must be "parent task" from the drop-down list.
If at any time one of your tasks has more than 5 subtasks, then given its complexity, it is advisable that this task be made a separate project.