To what does "the beginning" refer?

Mark 1:1 (ESV)

1 The beginning of the gospel of Jesus Christ, the Son of God.

That word can hardly be intended here to indicate the beginning of the book, that is, the first line. It is unusual and also superfluous to say at the beginning of a book that it begins with the first line! The phrase, “the beginning of the gospel” refers to something that follows. It amounts in some way or another to an announcement or a heading.1