Melchizedek is spoken about in Genesis ch. 14.
The name is said to be translated literally as "righteousness is my king", but also the translations of "king of righteousness/righetous king" are said to be possible as well.
He was the king of Salem and a priest who brought bread and wine, blesses Abram, and receives tithes from him. (verses 18-20)
He is referred to in Psalm 110:4 as well, speaking of being a priest after the manner of Melchizedek
By many Christians this is used as a reference that was foretelling that Christ was meant to be after the order long before His birth.
In the New Testament he is only referred to in the book of Hebrews, although extensively. He is mentioned in the verses of 5:6,10; 6:20; 7:1,10,11,15,17,21
In this passage Christ is referred to as a priest after the order of Melchizedek.
Many interpret this by looking back at where he is found in Genesis. In Genesis Abram tithes to Melchizedek, showing the order of Melchizedek to be higher than the Aaronic priesthood that came with the law.
In the same way, the verses in Hebrews shows Jesus to be of the order of Melchizedek, which was higher than the priesthood that came with the covenant made with Abraham. This sets Jesus as our High Priest forever.