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…为开端;以…作为开场白
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...进入正题之前,我先引用莎士比亚的几句诗作为开场白。The president prefaced his remarks by saying he has supported unemployment benefits all along.总统在发表讲话前说他一直以来都支持发放失业津贴。