During revision time, you will have to do small quizzes about your learned vocabulary. If you pass those quizzes, your vocabulary will be leveled up. On the other hand, if you fail to remember a particular word, that word will be pushed down to level 1, and you will have to level it up again in the next revision time.
There are 5 levels in total, words at lower levels will have more revision schedules than those at higher levels.
This feature helps you understand how Spaced Repetition works, and keep track your learning progress easily.