The Guardian explains the meaning of the title of one of Baha’u’llah’s Writings: The Hidden Words

With regard to your question concerning the meaning of the name "Hidden Words": It is, indeed, one of the most suggestive titles of the Writings of Bahá'u'lláh. These words are called "hidden" due to the fact that men have had neither the knowledge nor a true sense of appreciation of them before they were revealed by Bahá'u'lláh. It is through Him, Who is the sole Mouthpiece of God in this age, that spiritual realities and truths have been once more reinterpreted and revealed afresh to mankind. Bahá'u'lláh's message is thus the only key to a true understanding of the mysteries that envelop man's spiritual life. (From a letter dated 1 September 1935 written on behalf of Shoghi Effendi to an individual believer; Compilation ‘Hidden Words: References of ‘Abdu’l-Bahá and Shoghi Effendi’, prepared by the Research Department of the Universal House of Justice)