The author first describes the human high priest: he is taken from among men and appointed to act on their behalf in relation to God, to offer gifts and sacrifices for sins. He can deal gently with the ignorant and wayward because he himself is beset with weakness, and so he must offer sacrifices for his own sins as well. No one takes this honor for himself; he receives it when called by God, as Aaron was.
So also Christ did not glorify himself to become high priest, but was glorified by the one who said to him, You are my Son, today I have begotten you (Psalm 2), and in another place, You are a priest forever, in the order of Melchizedek (Psalm 110). In the days of his flesh he offered up prayers and supplications, with loud cries and tears, to the one who was able to save him from death, and he was heard because of his reverence. Although he was a Son, he learned obedience through what he suffered, and being made perfect he became the source of eternal salvation to all who obey him, designated by God a high priest after the order of Melchizedek.
The author wants to say more about Melchizedek, but it is hard to explain because the readers have become dull of hearing. By now they ought to be teachers, yet they need someone to teach them again the basic principles of God's word; they need milk, not solid food. Everyone who lives on milk is unskilled in the word of righteousness, since he is a child. Solid food is for the mature, who have trained their powers of discernment by constant practice to distinguish good from evil.