Action Items are relevant, current things that you are included in. You can view your "Action Items" on your homepage by clicking the "Home" button on the top right of your screen:
You can also sort action items by clicking on the tabs next to the "All" tab. When the "Incoming" tab is active, you will view all reviews in which you are a reviewer or an observer. The "Outgoing" tab will show all reviews in which you are the author. "Completed" will list recently completed reviews.
Your Action Item list is updated immediately if anything changes. You can refresh this screen manually; the home page refreshes automatically every five minutes.
When you have an Action Item directing you to commit files, you remove it by clicking the "dismiss" link on the left.
Action Items are also visible in other clients, but that is covered in the separate User Guides.