O come, thou Root of Jesse's tree

Latin translation of O Radix Jesse
O Root of Jesse, standing as an ensign among the peoples; before you kings will shut their mouths, to you the nations will make their prayer: Come and deliver us, and delay no longer.

Advent Carol
O come, O come, thou Branch of Jesse's tree, free them from Satan's tyranny, that trust thy mighty power to save, and give them victory o'er the grave. Rejoice, rejoice. Emmanuel shall come to thee, O Israel.

The antiphon today is based on the lineage of Jesus as the Messiah from the house of David (see Romans 15:12). Isaiah prophesied: "A shoot will come up from the stump of Jesse; from his roots a Branch will bear fruit." And, "In that day the Root of Jesse will stand as a banner for the peoples." (Isaiah 11:1, 11). The ancestry of Jesus, "Son of David" and thus also a shoot from Jesse (David's father), is recorded at the begining of Matthew's gospel and in Luke 3. He was born in Bethlehem, the David's home town, as Micah prophesied, saying, "Out of you will come for me one who will be ruler over Israel, whose origins are from of old, from ancient times." Micah 5:2. And Jesus is be beloved (the name David means "beloved"). Isaiah foretold of the Messiah, "So will he sprinkle many nations, and kings will shut their mouths because of him. For what they were not told, they will see, and what they have not heard, they will understand" (Isaiah 52:15).

See this page from Fisheaters for information about the custom of making a Jesse tree during Advent.