Spaced repetition originates from the way memory works. Initially, we need to make an early review of the learning material, to make sure it is retained in memory. However, as the material gradually gets consolidated, the review may be less and less frequent.
Most advanced applications of spaced repetition make use of computer software. Such software can collect information about student's memory and gradually adapt the optimum spacing of repetitions to individual needs. A typical optimization criterion used in spaced repetition is the requested level of knowledge retention, i.e. percent of knowledge that is to be remembered.
Surprisingly, humans differ little in the way their memory works. People with large differences in IQ will often exhibit similar spaced repetition patterns. For example, when we learn a foreign word, most people require a repetition within 1-10 days to ensure 95% retention. The second repetition may already come later. For example, between 2-25 days. After a few repetitions, well structured and meaningful learning material can be repeated with intervals lasting many years.
Practical applications of spaced repetition were first suggested in the book "Psychology of Study" by Prof. C. A. Mace in 1932.