Melchizedek is first mentioned in the Genesis 14:18. Abram returning from the defeat of the five kings, is met by Melchizedek, king of Salem, who brings wine and bread. The verses mention that Melchizedek is a priest to the Most High God and he blesses Abram.
The writer of Hebrews equates Melchizedek as a picture of the priesthood of Jesus. The writer's arguement is that Jesus and Melchizedek are both priests to God, yet neither is from the tribe of Levi, Melchizedek having been born prior to Levi and Jesus being decended from the tribe of Judah.