It'a a story about how Jesus came to the Americas after he rose from the dead and converted Native Americans who later lost the faith or died out.
Mormon was an angel/Indian who told this story to Joseph Smith in a vision in upstate New York in the 19th century.
Ironically, the original Mormons were communists and were persecuted for this {now they are supercapitalists and the richest religion per capita in the country).
They got chased out whereever they went and their men slaughtered, which is why they adopted polygamy to deal with all the widows).
They also became racists while living in Missouri during the border wars as a defense (they claimed black skin was the mark of Cain) to no avail as they got chsed out there as well.
They "evolved" this position after they began converting so many mixed folk in the 1970s.
They only denounced polygamy so that Utan could become a state.