Spaced repetition is second thing after active recall that makes learning more effective. Instead of re-reading your notes and blindly memorizing facts, if the content is studied through repetition in interval that makes a huge difference in retrieval capacity. When we start learning a topic, the the brain is charged with the information from what you are learning just after your study session has completed for that day. Over time, the brain starts losing that charge and the information starts diffusing away. Therefore to concentrate those information, we need to repeat what we had studied after a regular interval so that the decline of the information from memory never reaches zero and eventually flattens out.

The research paper that I talked about is linked below. Have a look if you want to.

https://drive.google.com/open?id=1QBvU7Pe8oTKTFDMMBitO89_8uqnBRlWi