For a range of interpretations of the Root of Jesse, maybe start with Wikipedia http://en.wikipedia.org/wiki/Tree_of_Jesse. Or just listen to the sung version and read the original Latin along with the English translation.
“O Radix Jesse, qui stas in signum populorum, super quem continebunt reges os suum, quem gentes deprecabuntur: veni ad liberandum nos, jam noli tardare”
translation:
“O Root of Jesse, sign for all peoples, in whose presence kings stand silent, before whom nations bow down in worship. Come to liberate us, do not delay.”
sung in chant:
http://www.youtube.com/watch?list=RDVRzOsCF6gSw&v=CvafrxZ_Ww4