Active recall is a principle of efficient learning, which says that we need to actively stimulate memory in the learning process. It is an opposite to passive review in which the learning material is processed passively (e.g. by reading, watching, etc.).

For example, when you read a text about George Washington, this is passive review.

If you answer a question "Who was the first US President?", this is active recall.

Active recall is many times more efficient in consolidating long-term memory. This is why just reading your study notes before exam is not likely to leave a long-lasting memory trace. On the other hand, if you ask your colleague to test you on the same material, the results will be better in the long run.

For more information on improving memory see Mnemonics.