The Kinderhook plates were a set of plates which Joseph Smith claimed he was translating, and were later found to be phony plates made by someone trying to test the mormon founder.
Joseph Smith’s own personal secretary recorded the fact that the mormon founder had “translated a portion” of the Kinderhook plates, and that Joseph Smith said:
“they contain the history of the person with whom they were found and 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.”
Kinderhook plates were later revealed to be plates that someone had made in order to trick Joseph Smith into pretending to translate phony plates, to expose him as a fraud and false prophet. The fact that he started “translating” them, and claimed their origins to be ancient, when they were phony, exposes the probability that Book of Mormon (in addition to evidence of plagiarism) and other works which Joseph Smith said he translated, such as the Book of Abraham (which was separately exposed as well), were not really translations at all.
The plates are still around today and were later tested, in 1980, using microscopy and various scanning devices to reveal they were not from the origins that Joseph Smith claimed they were. In 1981, the official magazine of the LDS Church ran an article stating that the plates were a hoax. In that article, they tried to cast doubt on whether or not Joseph Smith was actually translating them or not, but the fact remains that Joseph Smith’s personal secretary recorded Joseph Smiths own words that he was translating them, and had translated them enough to already knew what their contents were, even though the plates were completely bogus.