A preface is an introduction at the beginning of a book, which explains what the book is about or why it was written.
VERB
以…為開端；以…作為開場白
If you preface an action or speech with something else, you do or say this other thing first.
例句
I will preface what I am going to say with a few lines from Shakespeare...進入正題之前，我先引用莎士比亞的幾句詩作為開場白。The president prefaced his remarks by saying he has supported unemployment benefits all along.總統在發表講話前說他一直以來都支持發放失業津貼。