Your done cards list holds the cards you don't need to see again. As you work through the deck, clicking the Mark Done button moves a card to this page so you can focus on what you haven't fully learned yet. You can remove cards from your done cards list at any time to move them back into the deck for further quizzing.
Your starred items are bookmarked so that you can find them quickly. As you browse the PMBOK knowledge areas and processes, you may want to star the items that you need to study more. Then when you are done, you can unstar them to clear them off your todo list.