Completion is a percentage that represents the number of videos you have watched on a specific academic course. When you watch all the videos on a particular subject, you will achieve 100% completion for that subject.
Mastery, on the other hand, indicates your understanding of a topic or subject. It is measured by your performance on the end-of-lesson quizzes of all videos. If you get 100% correct answers on all quizzes, you will score 100% mastery.