Any line that starts with [] [ ] or [x] is a todo list item:
[ ] todo task
  1. todo task
[x] done task
  1. done task
You can mark tasks as done/undone by clicking on the corresponding checkboxes to the left.
All todo lists are hierarchical by default. It's enough to add several spaces before [] to move todo item into sublevel, like this:
[ ] parent item
space[ ] subitem
  1. parent item
    1. subitem
You can drag todo items to reorder or change their level. It works even on mobile devices, by the way.