“Messiah” (Hebrew: meshiyakh) or “Christ” (Greek: christos) means the Anointed One. It’s a reference to an ancient initiation rite of pouring oil (along with the authoritative words of institution) which formally appoints a man to an office. In the Old Testament, there are three offices into which chosen persons are anointed by God to serve his people in his name, i.e. the prophet, the priest, and the king.
In the New Testament, we see God the Son in the form of a human servant presented as God the Father’s ultimate and consummate prophet, priest, and king all wrapped up in one. Christ serves his people in all three capacities. He declares the counsel of God as prophet. He intercedes in the presence of God as priest. And he rules in righteousness before the face of God as king. Jesus the Son of God is the Threefold Christ.
Contemporary Evangelicalism typically speaks of the work of Christ in the categories of Lord and Savior. These dual roles certainly have their place in Scripture, especially as pairs in the Apostle Peter’s second epistle. But the Reformed tradition has found more utility in understanding the Son of God as the Anointed One according to his threefold function (Latin: munus triplex).