Testing an individual's memory makes the memory stronger and easier to retrieve. This is known as the testing effect. If a person is taught new material and tested on it a day after learning it the information will be more likely to be recalled when tested at a later date.
The timing of the testing is important - the information is more likely to be remembered if tested the day after learning rather than a week after learning. The testing effect is related to the forgetting curve which shows that the most rapid loss of information occurs within the first few days after learning the new information.
This effect shows the importance of reviewing and testing yourself on new concepts within a few days of learning.