If you want to remember those knowledge firmly for a long time, repetition is indispensable, but pure repetition (for example, repeated reading) does not work well. A better way to consolidate memory is "interval repetition." Because concentrated practice and repeated repetition can only form short-term memory, while interval repetition is more helpful to form long-term one. Therefore, after you have learned certain knowledge, as time pasts, you may forget part of them. Then you need to repeat the knowledge again to extract memories. This can re-trigger memories and consolidate what you have already know.