Re-ordering a task
You can find the handler on the left of the check box of the task (it consists of 3 white dots on a color line). Once you drag it you can drop it anywhere above or below the task. As you do so, you will see a preview of where it will be inserted.
Moving to a different checklist
You can even drag the task to a different checklist by dropping it on the left column on the target checklist
Moving to a sub-task
If you want to place the task under a different task (sub-task) simply stay on the same line and drag the handler to the right and drop it. You will see how it becomes a sub tasks as it is indented to the right. You can have as many sub-levels as you like but we recommend not using more than 3-4 sub-levels as it makes it difficult to work with (especially on mobile devices)