‘That’ is synonymous with Brahman, from the root ‘Brh‘, ‘To Uphold, Support’. Brahman is ‘That which upholds’, originally a Mantric expression for Yagnic formalities before it took on metaphysical meanings.

Para Brahman [the ‘Highest Expression’] of Brahman was as Nirguṇa Brahman: ‘Brahman without attributes’.

If something doesn’t have attributes you cannot express it. Except as Self-Negating Expression.