Melchizedek was the king of Salem and "priest of the most high God" who met Abram after a battle and blessed him, recorded in Genesis 14.
Genesis 14:18-20 in the King James Version reads:
18 And Melchizedek king of Salem brought forth bread and wine: and he was the priest of the most high God. 19 And he blessed him, and said, Blessed be Abram of the most high God, possessor of heaven and earth: 20 And blessed be the most high God, which hath delivered thine enemies into thy hand. And he gave him tithes of all.
These verses appear in Genesis 14, after Abram's rescue of his nephew Lot. Read it in context at Genesis 14:18-20, or the full chapter at Genesis 14. Encyclopedia entry: en.wikipedia.org/wiki/Melchizedek.