An authoress is a woman who writes books. The word is built the old way English used to mark “female” on job titles, like actress or waitress. You will still meet it in older novels, library catalogues, and some historical writing. Though actually, most people now just say author for a writer of any gender, and many writers prefer that. To some ears authoress sounds dated, and to some it sounds as if a woman writer were a special, smaller kind of author. It is not an insult by itself, but it is no longer the normal word, and in a school essay today author is the safer, kinder choice unless you are quoting an old text.