The Review training is based on the Ebbinghaus forgetting curve. The main principle behind this training is interval reviews.
If you've just learned a word in your main trainings, it will be available in review in 8 hours.
If you answer correctly in 8 hours, the interval before the next review will double, so 8 - 16 - 32 - 64 - 128 hours (you'll see this reflected in days). And this will happen each time you answer correctly. The maximum review interval is 364 days.
If you make a mistake, the training will give you the word one more time.
If you answer incorrectly just once, the word will come back to review in 8 hours and the intervals will start from scratch. If you answer incorrectly both times, the word will be labeled «Learning» and will pop up in all vocabulary trainings.
You can't have more than 128 words a day in this training. Words you've recently learned will pop up first, then the rest. If you practice all of the words today, you will get a new set tomorrow, provided there are words with matching intervals.