XP points (eXperience Points) are awarded each time you successfully answer a question, an exam, or a question session. XP points do not affect what you can do and are an optional metric.
Memory is a serious learning tool, and XP points only exist as a motivational goal to compete with your friends, classmates or other users. Certain learning modes, e.g. flashcards and multiple-choice earn less XP per answer than other modes such as typing.
You earn more XP for answering with high accuracy and/or speedily in relation to your normal speed.