Naskh and izaalah both refer to abrogation or changing of the ruling behind a verse or verses of the Qur'an, due to an evidence that was revealed later than it.
Naskh is mentioned in the Qur'an in 2:106 and 16:101. As part of understanding the Qur'an, it is necessary to not only accept naskh and mansookh, but strive to learn more about it. Therefore it is compulsory to learn the history of the Qur'an and its explanation by the Prophet, his Companions and their students.
Because the Qur'an was revealed over a period of 23 years, a number of varying situations and stages of human development are covered within its text and in the statements and actions of the Prophet. One of the benefits behind this is that it allows for the message of Islam to be applicable in all times, in all places and for all people.