Iteration means performing a task multiple times.
You can start several instances of a task at the same time, which is call multi-instantiation, or you can start another instance of a task when the previous one finished, which is called a loop. You can iterate a task or a call activity. If you have several tasks in sequence that need to be iterated, groups them into a subprocess and define the iteration on the call activity that calls the subprocess.
Use multi-instantiation if you want several instances of a task to be performed in parallel. For example, in a process for reviewing a document, you would issue the document to all reviewers at the same time, then collect all their feedback before moving to the next step.