According to the Twelver Shíʻah tradition, the twelfth Imám—known as the Mahdí, or “rightly guided one”—was born to the eleventh Imám, Ḥasan ʻAskarí, in 869 AD. After Ḥasan ʻAskarí died in 874, the Mahdí—who would have been five years old at the time—was purportedly hidden to protect him from the ruling ʻAbbásid caliphate. He would communicate with his followers through a series of four intermediaries. This period, known as the Minor Occultation, ended in 941 with the death of the fourth intermediary. Since that time, Twelver Shíʻahs believe that the Mahdí has continued to exist in hiding, and will eventually reveal himself to battle the forces of evil and firmly establish submission unto God—the literal definition of “Islám”—as the universal religion for mankind.
Moojan Momen refers to this tablet in his article, “Shi`i Islam” (1995; available online), as evidence that Baha’u’llah rejected the story of the twelfth Imám as a pious fraud.
For more information on the concept of the Mahdí, refer to the following articles in the Encyclopedia Iranica:
2. “Ḡayba”: http://www.iranicaonline.org/articles/gayba
 The brother of Imám Ḥasan ʻAskarí, known to Twelver Shíʻahs as "Jaʻfar-i-Kadhdháb" (“Jaʻfar the Liar”). In this tablet, Baha’u’llah dismisses this tenet of Shíʻah eschatology by establishing the truth of Jaʻfar’s reply to the question about his brother’s son. (This is not the same person as Imám Jaʻfar al-Ṣádiq, who lived in the eighth century AD—roughly a hundred years before Jaʻfar-i-Kadhdháb.)
 This characterization of Jaʻfar is likely a reference to the fact that generations of Twelver Shíʻahs have considered him a liar, even though he was actually telling the truth.