The opening verse is the title to the whole book, and it introduces us to the content of the book. The verse also tells us some important things about how we should read the book.
The title tells us that the writing that follows is a song. It is poetic language put to music, or lyric poetry. Which means we must read it as poetry and not as some other type of writing such as narrative or prophecy.
The title also introduces the book much like Proverbs 1:1 and Ecclesiastes 1:1 introduce those two books. The title therefore invites us to link this book to the other wisdom books that are associated with Solomon.1
1 The Song of Songs, which is Solomon’s.