Each word has its own date: just before you would forget it.
After every answer the app works out the word's next date from how you answered. A quick, sure answer moves the word further out; a slow one brings it back sooner, and a word you could not say comes back soonest.
The more often you remember a word, the longer the gaps grow — days, then weeks, then longer. This is spaced repetition, planned by FSRS, a memory model built on research.
