What is the explanatory section at the begging of a book called?

Answer 1

Prelude

you mean Preview, a prelude refers to music

Well, actually, I think you mean the Prologue. Books can begin with Prologues which is a little section or paragraph at the beginning of a book. Some books also contain an Epilogue which is an added section or paragraph to the end of a book which isn't a chapter but just an add on.

Answer 2

The explanatory section at the beginning of a book is called the preface. It typically provides insight into the author's motivation for writing the book, background information, and acknowledgments. It helps set the tone for the reader before delving into the main content of the book.

Answer 3

the foreword