What is spaced repetition?
Spaced repetition is an algorithm that helps you learn more efficiently. How often Easy Hanzi quizzes you on a card depends on how well you know this card. The more you know a card, the more the interval between two quizzes increases. Conversely, the less you remember a card, the more frequently Easy Hanzi will quiz you.
For example, let's say you are quizzed on the card "你好": if you get it wrong, it will be asked again the next day. If you don't remember it again on that day, you will be asked again the next day and so on.
However, if you get it right, Easy Hanzi will only quiz you on that card 2 days later; and if you get it right again, you will be quizzed only 5 days later, then 10 days, 15 days, etc. Basically, the more you get a card right, the less often you will be quizzed. This is why on each new session, you will see the number of cards in the session go down every day. For example, on the "1000 most common characters" deck, on the first day there will 1000 cards, on the second day maybe 983, then 961, etc. (it's not a bug, it's the spaced repetition algorithm at work!).
Eventually, when you reach the point when there's no more cards to learn, it means you know this deck particularly well - and can move on to the next one.