The name Melchizedek (מַלְכִּי־צֶדֶק or מַלְכִּי־צָדֶק) which means King of Righteousness, occurs just two times in Tanakh. In Lech Lecha Melchizedek brought bread and wine to Abram after his victory over the four kings. Melchizedek is identified as the king of Salem - which is Jerusalem, then a Canaanite city.
He blessed Abram in the name of "El Elyon" (Most High). Then he gave him a tenth of everything (a translation which preserves the ambiguity of the original).
It would appear that this tithe, if given by Abram to Melchizedek, indicates that Abram accepts Melchizedek’s priesthood. It is not clear whether “God Most High” (El Elyon) refers to the very God of Abram’s covenant, or whether Abram recognizes the validity of another god. "If it is the former, it is hard to claim that Abram “invented” or “discovered” monotheism."
Rubens, The Meeting of Abraham and Melvhizedek, c. 1625
The second time the name Melchizedek is mentioned is in Psalms 110: Thou art a priest for ever after the manner of Melchizedek.
According to the Talmud (Nedarim 23), Melchizedecek is the sam as Shem. Rashi on Bereshit 14:18 says Melchizedek is Shem, the son of Noah. Noah is the ancestor of Levi, who was the father of Aaron and Moses. Thus Melchizedeck is an early precursor to the preistly lineage of Aaron and Moses.
Click here for an excellent post that explicates and criticizes the Christian myth of Melchizedeck.