Passive review is the opposite of active recall, in which the learning material is processed passively (e.g. by reading, watching, etc.).
For example, to improve memory through passive review, an individual may read a text today; to not forget it, it is repeated tomorrow and then 4 days later and then 8, 16, 32, 64, etc., days later. They don't ask themselves to explain the content of the text, but only reread the content. If they think to recall something, they are more likely to keep it in their memory. Passive review is a simple method but it is not as effective. Active recall is more complicated and difficult (because it forces one to recall something) but it is highly effective.