Melchizedek is a person who appears in a short passage in the book of Genesis and blesses Abraham. In exchange, when Abraham is victorious in battle (rescuing his nephew Lot in the process), he gives him a tithe of the spoils of war.
Who Melchizedek is remains a mystery, as outside of Scripture there is little or no reference to him. One teaching is that Melchizedek is a pre-incarnate appearance of Jesus Christ.
The Epistle to the Hebrews draws a comparison between Melchizedek and the priesthood established by Aaron to explain how Jesus was able to make a sacrifice in place of all mankind's sins, circumventing the law of Moses.