Passive review is the opposite of active recall

Active recall

Active recall is a principle of efficient learning, which claims the need to actively stimulate memory during the learning process. It contrasts with passive review, in which the learning material is processed passively . For example, reading a text about George Washington, with no further action,...

, in which the learning

Learning

Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...

 material is processed passively (e.g. by reading, watching, etc).

For example, to improve your memory

Memory

In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

through passive review, you learn in this way: you read a text today; to not forget it, you repeat it tomorrow and then you repeat 4 days later and then 8, 16, 32, 64, etc., days later. You don't ask yourself to explain the content of the text, but only reread the content. If you think to recall something, you are more likely to keep it in your memory. Passive review is a simple method but it is not as effective. Active recall is more complicated and difficult (because it forces you to recall something) but it is highly effective.