This week's Audanika Harmony Theory lesson continues with intervals. If you haven't already, you should go read Lesson 4, which introduces the concept and names of intervals. This one gets into Major and Minor intervals, so it will help to review Lesson 4. Seriously. It only takes 5 minutes to read and intervals are something everyone should understand.
When you've done that, watch this video and then head over to Lesson 5.