Untrained simply means the artist didn't have any formal art training, such as going to an art school or institute. In fact, Outsider art is a current known for having little to no contact with the "mainstream" art world, with these artists having never been to an art school and even sometimes rejecting formal art training. Some great examples of "untrained" artists are famous painters such as Henri Rousseau and Vincent van Gogh
Ummm, I think you wanted to add options but something that could be used to help clarify is rereading or finding words that you dont understand and search them in a dictionary.
