Ebbinghaus discovered the effect while memorizing nonsense syllables: repetitions spread over days required less total effort for the same retention than repetitions crammed together. More than a century later, Cepeda and colleagues synthesized the accumulated verbal recall literature and confirmed the pattern quantitatively, adding a further wrinkle: the best gap between study sessions grows with how long you need to remember the material.
Why spacing works is still debated. Leading accounts point to the extra retrieval effort each spaced session demands, reduced redundancy in how the memory is encoded, and the benefit of re-encoding material in a slightly different mental context each time. Whatever the mechanism, cramming optimizes for tomorrow's test and spacing optimizes for the months after.
The spacing effect is the principle; spaced repetition is the practice built on it. For a reader, the cheapest application is simply to revisit saved material on a schedule, letting old highlights resurface days or weeks later instead of vanishing into an archive.