It's a statement of absolute apophaticism – as I understood it, one can speak of the Brahman of Shankaracharya, the Gottheit of Meister Eckhart, the Tao of Lao Tzu, the One of Plotinus, or al-Dhat of Ibn ‘Arabi... that which is Absolute and Infinite; Uncreated and Unconditioned, the One in which is All-Possible and contains all within Itself without distinction or differentiation.
It is absolutely indeterminate – any positive definition is a determination and delimitation.
The essentially apophatic definition cannot be rationally contested, nor is it open to rational proof or argument.
It is without any limitation, restriction, or determination, it is absolute totality, for if anything were exterior to it, it would not be Infinite.
That said, again as I understand it, in Advaita Vedanta, Ishvara is a manifested form of Brahman, what one might term an upaya, as 'skillful means' to 'come near or come towards' an absolute apophatism.