Creating Custom Fail-fast Iterator?

As part of a homework assignment in my 1st Java Class, I am creating my own Circular Generic LinkedList and Array class. My class uses the Queue Interface Extends Iterable but I am creating my own methods to work with. For the most part, I believe I have been successful in creating the class aside from one method. That method is the Iterator<E> iterator().

/**
* Return a fail-fast iterator, which throw a java.util.ConcurrentModificationException
* if the client modifies the collection (via enqueue(...) or dequeue()) during iteration.
*/
@Override
public Iterator<E> iterator() {

I don't understand how an iterator, let alone a "fail-fast" iterator ties into my project. I've spent hours reading up on a way to imploy my own generic fail-fast iterator but to no avail.

I feel like I could come up with some workable code if I knew what the point is to useing a user-defined, non Java Library iterator is to do.

As well, does throwing a ConcurrentModificationException require a try and catch block?