Herein is a formal, mostly legal or official word meaning “in this” — in this document, this letter, this set of rules, or this situation. A contract might say “the rules herein,” meaning the rules inside this paper. You almost never hear it in ordinary talk; people just say in this letter or in this book. Though actually, students who already know here, there, and where may think herein is only a fancy here. The in is doing real work. It means inside this text or this matter, not “at this spot on the street.” That street meaning is here or hereabouts. A cluster of old paper words is built the same way: hereinafter, hereof, hereto, herewith. They all sound stiff on purpose. They belong to documents that must not be fuzzy.