Al-Ahqaf: (21) "And mention the brother of 'Ad..."
Al-Ahqaf: The plural of hiqf, which is an elongated, elevated sand dune that has a curve, derived from iḥqawqafa (when a thing becomes crooked). The 'Ad were people of pillars who lived among sand dunes overlooking the sea in a land in Yemen called al-Shiḥr. It is also said: between 'Oman and Mahrah.
{The warners (al-nudhur)}: The plural of nadhīr, meaning the one who warns or the act of warning.
{From before him}: From his past.
{And from behind him}: And from his future. It is also recited: "from before him and from after him."
The meaning: Hūd (peace be upon him) warned them, saying: "Do not worship except Allah; indeed, I fear for you the punishment." He informed them that the messengers who were sent before him and those who would be sent after him were all warners, just as he was a warner.
Ibn 'Abbās (may Allah be pleased with him) said: It means the messengers who were sent before him and those who were sent during his time. The meaning of "from behind him" according to this interpretation is: from after his warning.
This is if you connect it to "he warned his people." You may also consider the words of the Exalted, {And warners have passed on before him and after him}, as a parenthetical clause between "he warned his people" and {that you not worship}.
The meaning becomes: Mention Hūd’s warning to his people regarding the consequences of polytheism and the great punishment; for the messengers who preceded him and those who came after him had warned of the same, so remember them.
{They said: "Have you come to us to turn us away from our gods? Then bring us what you promise us, if you should be of the truthful."}