At the end of the lessons you select how well you understood the lesson. However, unless there is some sort of feature that allows you to review lessons you didn't understand as well, i don't know why that feature would be there. I'd looove to be able to review a lesson I might not have understood well at the time but its hard to find older lessons, and I think it would be awesome to implement a "review" old lesson feature that you selected "not well understood" on, similar to how the flashcards work. Thank you so much for all your hard work ! :)