Lets you mark tasks as not done. Maybe your boss was rather unimpressed with your performance, or you accidentally marked a task as done. It's okay, you humans make mistakes.
You can also use undone /search terms to save a step in marking multiple tasks as not done. If the search returns multiple tasks, then it'll ask for confirmation so you don't accidentally mark unrelated tasks as not done. Just say y to mark all the tasks as unfinished; anything else is interpreted as a fresh command.
See also help context if you're wondering how the last example works!
Examples: