This statement means that all that Allah willed to exist must exist in the time that Allah willed for it to exist. Included in this meaning is that Allah Willed the good, evil, obedience, disobedience, blasphemy and belief. What Allah did not will to exist shall not come into existence – It will not happen and will not occur.
The Will of Allah is eternal, everlasting and does not change. The above statement is taken from the sayings of the Prophet, salla-l-lahu ^alayhi wa sallam. Abu Dawud narrated in his book ‘as-Sunan’ that the Prophet, salla-l-lahu ^alayhi wa sallam, taught this statement to his daughter:
روى ذلك أبو داود “ما شاء الله كان وما لم يشأ لم يكن””
“Whatever Allah willed to be shall be and whatever Allah did not will to be shall not be”.