The phrase “Root of David” refers to Jesus Christ in the book of Revelation (Revelation 5:5; 22:16). The word “root” usually means “source.” However, in this phrase, it describes Jesus as a descendant of King David. Revelation 22:16 also calls Jesus David's “offspring,” meaning someone from his family line. Jesus came from David's family, just as a branch grows from the roots of a tree (compare Isaiah 11:1).
See also Root of Jesse.