The introduction to a book or talk is the part that comes at the beginning and tells you what the rest of the book or talk is about.

If you refer to a book as an introduction to a particular subject, you mean that it explains the basic facts about that subject.

You can refer to a new product as an introduction when it becomes available in a place for the first time.

If you say that someone or something needs no introduction, you mean that they are so well known that everyone knows who or what they are.