Hi. Not much is known of Melchizedek outside of Genesis 14. Here's what we do know about him: 1. He was king of Salem (later known as Jerusalem). 2. He was priest of the Most High God (El-Elyon).
In Psalm 110:4, King David prophesies of the coming Christ, and declares God's promise that He (the Messiah) will be "a priest forever after the order of Melchizedek". Later, the writer of Hebrews would discuss the great significance of this prophecy in chapters 5, 6, and especially 7, to demonstrate that Jesus' eternal priesthood is better in every way to the temporary Levitical priesthood.
In Hebrews 7:3, the writer notes that Melchizedek was "Without father, without mother, without descent, having neither beginning of days, nor end of life; but made like unto the Son of God; abideth a priest continually." In other words, he had no recorded genealogy. As we've seen from Genesis 5 onward, the Bible makes a big deal about recording genealogy. Elsewhere in Scripture, genealogies are recorded even for foreign rulers outside of Israel. So for Melchizedek not to have a record, it is noteworthy. And because of this, the Bible uses him as a type and foreshadow of Jesus Christ. Not that Jesus didn't have an ancestral record; Matthew 1 and Luke 3 lists his very detailed lineage! But rather, Melchizedek is used as a type or figure of an eternal priesthood, of which Jesus Himself is the fulfillment of that type and figure, being a High Priest forever who ever makes intercession for us (7:25).
One note: in verse 3, when it says that Melchizedek was "made like unto the Son of God; abideth a priest continually", it's important to understand what the original Greek word was implying. Afoomoiooménos, which the KJV translates as "made like unto", means to represent as a copy or facsimile; a resemblance. It is not implying that the man Melchizedek actually lived forever and is still living forever. Rather, it's the office of his priesthood that is being used as the type or foreshadow, not the human being himself. Hope that all makes sense! :)