The forgetting curve is a mathematical formula that describes the rate at which something is forgotten after it is initially learned.
This is comprehensive enough. Simply put, information retained reduces with time and review will help in retaining it. Moreover, the frequency of review can be reduced as the information loss will become lower but requires the interval between reviews. The important thing to remember is this image
Anki - Simple. Many types of questions. Although I found cloze deletion to be much effective. They do the heavy lifting in terms of remembering when to surface up the question considering last time it was reviewed.