"And he says in another place, 'You are a priest forever in the order of Melchizedek.'" NIV translation
Paul reminds us of the first high priest which was Melchizedek. We were introduced to him in Genesis 14 when he blessed Abram but there is not much known about him.
"During the days of Jesus' life on earth, he offered up prayers and petitions with loud cries and tears to the one who could save him from death, and he was heard because of his reverent submission. Although he was a son, he learned obedience from what he suffered and, once made perfect, he became the source of eternal salvation for all who obey him and was designated by God to be high priest in the order of Melchizedek." NIV translation
Jesus was a man of prayer while He walked this earth and His Father heard. He submitted to the will of the Father in going to the cross on our behalf. In His obedience, He became our High Priest as appointed by the Father.
"We have much to say about this, but it is hard to explain because you are slow to learn. In fact, though by this time you ought to be teachers, you need someone to teach you the elementary truths of God's word all over again. You need milk, not solid food!" NIV translation
Paul goes on to say that there is much more to teach about this but it is for the Christian that is maturing. He contrasts the word of God to food for our body and explains that they still require milk like a baby. He also says that by now they should be able to teach others but they are not. Although this is not as important today as it was at the time, we should all be maturing and able to tell others about our faith. During Paul's time, it was vital because people did not have the scriptures on paper, their phones, or anything else.
"Anyone who lives on milk, being still an infant, is not acquainted with the teaching about righteousness. But solid food is for the mature, who by constant use have trained themselves to distinguish good from evil." NIV translation
So, how do you mature as a Christian? Paul tells us that it is by constant training in the Word. This can take various forms but the key point is that it is a daily part of our lives. Why is this important? We see that study in the Word of God is how we train ourselves to tell the difference between good and evil. That is probably the greatest failing in the church today as most Christians do not study enough to know what the Bible says about what is good and what is evil. In our society today, it seems like we have a majority of people trying to convince us that good is evil and evil is good. Without the solid food of the scriptures, we will go along with whatever the rest of the world says instead of what God says.