[Explanation of: “The Night of Decree is better than a thousand months.”]

Answer—and Allah is the Granter of success: Its explanation is clear: the sūrah, from beginning to end, speaks about the virtue of the Night of Decree and the virtue of the Qur’an.
The meaning of its being “better” is that obedience to Allah the Exalted therein—and worship of Him—is better than worshipping Him for a thousand months other than it. That is, Allah knows best, the good deed therein is multiplied manifold. If, outside the Night of Decree, a good deed is [rewarded] tenfold; and a thousand months are approximately thirty thousand nights; and a good deed on each night is [rewarded] tenfold; the total would be three hundred thousand good deeds.
On this basis, a single good deed on the Night of Decree is better than three hundred thousand good deeds. And Allah knows best. Praise be to Allah, Lord of the worlds; and may Allah bless Muḥammad and his family and grant them peace.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1