-There are certain rectifications that could only be done by someone with the position of high priest.
-At every period in time there is one person who represents the concept of high priest and can do these rectification.
-The height of these rectifications was done, when the high priest was able to do his work in the temple.
-Adom originally had this as part of his job. Eventually it got passed down to Noach who gave it to his son Shem.
-After the war of the four kings Malki Tzedek (Shem) came to greet Avraham 
- He made a mistake of putting Avroham before G-D, the position got taken away from him.