Melchizedek was the greater priest...foretelling of Jesus

Genesis 14
8Then the king of Sodom, the king of Gomorrah, the king of Admah, the king of Zeboyim and the king of Bela (that is, Zoar) marched out and drew up their battle lines in the Valley of Siddim 9against Kedorlaomer king of Elam, Tidal king of Goyim, Amraphel king of Shinar and Arioch king of Ellasar—four kings against five. 10Now the Valley of Siddim was full of tar pits, and when the kings of Sodom and Gomorrah fled, some of the men fell into them and the rest fled to the hills. 11The four kings seized all the goods of Sodom and Gomorrah and all their food; then they went away. 12They also carried off Abram’s nephew Lot and his possessions, since he was living in Sodom. 13A man who had escaped came and reported this to Abram the Hebrew. Now Abram was living near the great trees of Mamre the Amorite, a brother[b] of Eshkol and Aner, all of whom were allied with Abram. 14When Abram heard that his relative had been taken captive, he called out the 318 trained men born in his household and went in pursuit as far as Dan. 15During the night Abram divided his men to attack them and he routed them, pursuing them as far as Hobah, north of Damascus. 16He recovered all the goods and brought back his relative Lot and his possessions, together with the women and the other people. 17After Abram returned from defeating Kedorlaomer and the kings allied with him, the king of Sodom came out to meet him in the Valley of Shaveh (that is, the King’s Valley). 18Then Melchizedek king of Salem brought out bread and wine. He was priest of God Most High, 19and he blessed Abram, saying, “Blessed be Abram by God Most High, Creator of heaven and earth. 20And praise be to God Most High, who delivered your enemies into your hand.” Then Abram gave him a tenth of everything.

Melchizedek is a reference to Jesus Christ who gave us bad and wine (like Melchizedek) to remember His sacrifice for our sins. Jesus Christ is our mediator as Melchizedek was Abrams.

I can't summarize Jesus Christ's role as our mediator any better than with this Puritan Prayer.

Glorious Jehovah, My Covenant God,

All Thy promises in Christ Jesus are yea and amen, and all shall be fulfilled. Thou hast spoken them, and they shall be done, commanded, and they shall come to pass. Yet I have often doubted Thee, have lived at times as if there were no God. Lord, forgive me that death in life, when I have found something apart from Thee, when I have been content with ephemeral things.

But through Thy grace I have repented; Thou hast given me to read my pardon in the wounds of Jesus, and my soul doth trust in Him, my God incarnate, the ground of my life, the spring of my hope. Teach me to be resigned to Thy will, to delight in Thy law, to have no will but Thine, to believe everything Thou doest is for my good.

Help me to leave my concerns in Thy hands, for Thou hast power over evil, and bringest from it an infinite progression of good, until Thy purposes are fulfilled.

Bless me with Abraham’s faith that staggers not at promises through unbelief. May I not instruct Thee in my troubles, but glorify Thee in my trials;

Grant me a distinct advance in the divine life; may I reach a higher platform, leave the mists of doubt and fear in the valley, and climb to hill-tops of eternal security in Christ by simply believing He cannot lie, or turn from His purpose.

Give me the confidence I ought to have in Him who is worthy to be praised, and who is blessed for evermore.

Taken from ‘Valley of Vision: A Collection of Puritan Prayers’, edited by Arthur Bennett