A designation of the future Messianic king.
God says: "I will raise up to David a just branch: and a king shall reign, and shall be wise: and shall execute judgment and justice in the earth" (Jeremiah 23).
The use of this expression for the future son of David is probably suggested by Isaiah 11, where the Messias is represented, "a rod out of the root of Jesse, and a flower shall rise up out of his root."
The ideal king of the future will be characterized by justice and deserves to bear the name "The Lord our just one" (Jeremiah 23).
New Catholic Dictionary