The Hebrew word kohen refers to a priest — one ordained to serve in sacred duties before God on behalf of the people. It is one of the most important titles in the Old Testament, appearing over 750 times. The Aaronic priesthood (Exodus 28) was consecrated to offer sacrifices, tend the tabernacle/temple, pronounce blessings, and teach the Torah. The High Priest (kohen ha-gadol) alone entered the Holy of Holies on Yom Kippur to make atonement. Melchizedek (Genesis 14) was a mysterious priest-king of a higher order — the type Christ ultimately fulfills.
The entire Levitical priesthood points to Jesus Christ. The author of Hebrews devotes five chapters (Hebrews 5–10) to demonstrating that Christ is the ultimate High Priest — not after the order of Aaron, but after the order of Melchizedek (Psalm 110:4; Hebrews 7). Christ offered not the blood of animals, but His own blood — once for all (Hebrews 10:10). He is both the Priest and the Sacrifice. Through His priestly work, believers are made a royal priesthood (1 Peter 2:9) — mediators of God's grace to the world.