The book of Hebrews was written to explain that Jesus Christ was all the Jewish believers needed - he was the best, far superior to their old Jewish religion.
Hebrews probably was written before the destruction of the Temple in Jerusalem in A.D. 70. It was written to Jewish Christians in Jerusalem and throughout the world.
Important people in this book are the Old Testament men and women of faith mentioned in chapter 11.
What would you think of a boy your age who wore a diaper to school, crawled from class to class, threw tantrums when he didn't get his way, sucked on a pacifier, ate strained carrots and creamed spinach, and drank milk out of a bottle?
Someone acting like a baby at school, in a store, or at work would be weird. But often people are babyish in their spiritual lives. For Christians who have been in God's family for years to still act like infants - well, there's just no excuse. In the same way that little kids grow up physically, "baby" Christians need to grow up in their relationship with Christ.
That's what the book of Hebrews is all about. The author (we don't know who he or she was) wrote to Jewish believers, "Hebrews", who were immature in their faith and didn't understand that Christ was all they needed.
Whether you've been a Christian for a week or for several years, you need to listen to the message in this book. Read it and decide to grow up in your relationship with Christ.