The foundation, origin, or roots of something, especially an artistic work or cultural phenomenon; the original source or inspiration for many similar or related things that followed it. The term was originally applied to the earliest versions of scripture, literary works, and musical compositions but is now used more broadly. I think The Catcher in the Rye can be seen as the urtext for the many novels about disillusioned teens that followed it. The urtext of the movement seems to be a speech he gave 10 years ago.