Hoax, a translation was never made, and a statement attributed to Joseph about what they were was from someone elses journal. No translation was done, though the Church had them for many years. They still might.
But no one knows? Quite telling this.
JOSEPH PRONOUNCED THESE PLATES GENUINE!
As described in detail with annotations HERE, Joseph Smith accepted these plates as authentic and even claimed he had translated a portion of them.
According to the diary of William Clayton, Joseph Smith's private secretary:
I have translated a portion of them and find they contain the history of the person with whom they were found. He was a descendant of Ham, through the loins of Pharaoh, king of Egypt, and that he received his kingdom from the Ruler of heaven and earth. Joseph SmithThe plates were lost for a number of years until one was rediscovered in the 1950's in Chicago Historical Society Museum.
In 1962, Welby W. Ricks, President of the BYU Archaeological Society, hailed the find as a vindication of Joseph Smith's work:
A recent rediscovery of one of the Kinderhook plates which was examined by Joseph Smith, Jun., reaffirms his prophetic calling and reveals the false statements made by one of the finders....In the mid-1960's, an engineering analysis conclusively proved the plates were of 19th century origin. They were part of a cruel hoax, and Joseph Smith was duped.The plates are now back in their original category of genuine.... Joseph Smith, Jun., stands as a true prophet and translator of ancient records by divine means and all the world is invited to investigate the truth which has sprung out of the earth not only of the Kinderhook plates, but of the Book of Mormon as well.
The Mormon Church has since adjusted its position on the plates in line with your statement.