An action item is simply a follow-up task that can be assigned to yourself or another employee. You will receive an email when a task is due, when a task is assigned, or when a task you have assigned to someone else is completed.
In the example below an up-line supervisor has reviewed an entry and notices that although the documentation is good the supervisor has not set a task for a review to make sure the problem is corrected. He quickly creates a task for the supervisor to follow-up and document the conversation. The supervisor will get an email that the task has been assigned and will also get an email when the task is due.