Via dolarossa: Great High Priest is what Jesus became to us when he was seated at the right hand of God. Before Jesus we were separated from God by sin, and had no way to approach God except through animal/food sacrifices which were only temporary. With Jesus' sacrifice, He became our intercessor with God so that we could now approach God through Jesus. So the term Great High Priest is in reference to the Son of God, Jesus of Nazareth - He who died for us and was sacrificed for our sins.
I have been watching on TV this Jewish Rabbi who is a born again christian. He is so wonderful. He breaks down everything in the Old Testament and relates it all to the New Testament and he explains everything about the Jewish people and gentiles. It's fascinating.