To set a task list to be grouped, in the task list section of the app, go into a task list's settings and then check the "Group task list" switch like this:

If a task list is set as grouped:

The task list name is displayed at the top.
Grouped task lists are collapsible.
Tasks are locked together (ie. it is not possible to change the order of the tasks in grouped task lists).
It is possible to delete an entire task list at once.
When hovering over the list, the number of completed tasks vs uncompleted tasks will show in a white bubble.
Once all tasks within the list are completed, the task list name will be crossed out.

 If a task list is set as ungrouped:

• Tasks within a task list are added is individual entities (No task list header is shown).
• They can be reordered in any order (Tasks are not locked together).
• It is not possible to delete an entire task list at once.