It is obvious that for there to be a new convenat there must have been an old one. I explain a little about the old covenant: This was about offering sacrifices for sins through certain animals, and certainly these sacrifices were never enough, since they had to be done continuously. On the other hand, there was also a wall between God and man, which was created at the time of man to sin, and there was a high priest who was the person who used to approach to God.
Now, that covenant that we just explained was replaced by a new one, a better one, a perfect one, which allows us to approach God without any problem and makes us heirs of Him.
The author of the Hebrews speaks to us about it and says: