You’ll remember material more reliably if you study it on many occasions, with some space in between—rather than if you spend the same amount of time cramming it all in one evening.
Your likelihood to forget some material follows an exponential decay curve, first characterized by Ebbinghaus in the mid-19th century. Reinforcing material immediately isn’t very useful because you’re unlikely to have forgotten it: Less accessible memories are more reinforced by retrieval. If you wait a while, you’re more likely to have forgotten it.
Each successive reinforcement appears to flatten that forgetting curve, so you can wait longer and longer between each review. The dynamics of that flattening effect are not well characterized (for one model see Two-component model of memory), but Spaced repetition memory system algorithms have used constant factors with relative success, which creates an exponentially-increasing series of test intervals, e.g. 5 days, 2 weeks, 1 month, 2 months, 4 months, etc.
Kornel (2009), in an experiment involving GRE-type vocabulary:
Combining the three experiments, 90% of participants learned more in the spaced conditions than the massed conditions, whereas only 6% of participants showed the reverse pattern.
Drawn from Nate Kornell (2009):
Branwen, G. (2009). Spaced Repetition for Efficient Learning. Retrieved December 16, 2019, from https://www.gwern.net/Spaced-repetition
Kornell, N. (2009). Optimising learning using flashcards: Spacing is more effective than cramming. Applied Cognitive Psychology, 23(9), 1297–1317. https://doi.org/10.1002/acp.1537