These chants and texts are some of the most treasured songs of Christianity. Jesse was the father of David, and the Messiah was expected to come from the family of David. "Radix Jesse" is usually translated "Root of Jesse." I've also seen it translated "sprout of Jesse," but seems to stretch a bit far, and it certainly isn't as poetic. Also "rod of Jesse," and THAT might garner some interesting comments around here.
Also see Flos De Radice Jesse and Lo How a Rose E'er Blooming.
-Joe Offer-