Hibi uses spaced repetition (a modern algorithm, FSRS): it tries to show you each word just before you would forget it. Every time you answer, it notes how hard it felt and recalculates the date of the next review. The better you know a word, the longer the intervals grow (a few days, then weeks, then months). For the detail of the principle, see the blog.
The daily goal (adjustable, about 20 by default) is a pace marker, not a wall: nothing locks up whether you do more or less. It is a hint to help you keep a steady rhythm, never a paywall. The streak counts the consecutive days you have studied: if you skip a day, it resets to zero, but your progress and learned words stay intact.
A word becomes mastered when you have recalled it correctly several times in a row and its review interval has grown long enough (on the order of several weeks). Hibi then keeps showing it to you, but much more rarely, just enough to keep long-term memory alive. Nothing is ever “finished” for good: if you stumble later, the word comes back more often.
Nothing special to set up: just say it was hard when the word comes back. Hibi then shortens its interval and brings it back sooner and more often, until it finally holds. A stubborn word is not a failure, it is exactly what the algorithm is built to catch up on.
No. Hiragana and katakana have their own tracking, separate from vocabulary and grammar. Hibi remembers the kana you hesitate on and offers to review them on their own, without mixing them into your word reviews. That way, the vocabulary SRS stays focused on the words themselves.
Nothing serious. Overdue reviews simply wait for you the next day, they do not disappear. If the pile has grown a lot after a break, pick it back up at your own pace: do what you can each day and the algorithm naturally spreads the rest out again. A few regular minutes beat one big occasional session.
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