Lets you mark tasks as done. Wouldn't be much of a todo list without this one!
You can also use done /search terms to save a step in marking multiple tasks as done. If the search returns multiple tasks, then it'll ask for confirmation so you don't accidentally mark unrelated tasks as done. Just say y to mark all the tasks as finished; anything else is interpreted as a fresh command.
See also help context if you're wondering how the last example works!
Examples: