Complex sentence: National Ambassador for Young People's Literature is a literary honor presented bi-annually by the Library of Congress to an author or illustrator who is a U.S. citizen and who has made a substantial contribution to young people's literature.
Split into simpler sentences: National Ambassador for Young People's Literature is a literary honor chosen bi-annually by the Library of Congress. It is given to an author or illustrator who is a U.S. citizen and has made a substantial contribution to young people's literature.