represents one box.
It contains cue cards for every word (in the first box are 4 cards, in the second 0, and so on).
When you answer correctly, the corresponding card is put into the next box.
But if you get it wrong, it falls all the way back to the first box.
The currently active box is higlighted.
When there are no more vocabs in any box, you're done and your brain has memorized it.