All is absolute, pure, infinite consciousness, non-dual, supreme intelligence, the self-existent Self or Brahman.
Maya or illusion, the powers of veiling and projection are inherent powers in Brahman. These powers manifest an apparent but unreal universe.
It is unreal because it was not before manifestation and will not be after dissolution.
Therefore it is likened to a dream in the supreme intelligence or mind of Brahman.
Thus the apparent universe is but an appearance based on Brahman. It does not exist apart from Brahman.
Source – article titled Is Life A Dream? By Alan Jacobs in May 2016 edition of Prabuddha Bharata.