A root text is a poem meant to be memorized and to be supplemented by a written or oral commentary.
It is a "word commentary," one that paraphrases and expands the root text, which because of its brevity can be rather like a telegram.
Root-text or core-text, sometimes in verse and sometimes in prose, surrounded by concentric layers of prose commentary , and sub-commentary.
The roottext is usually relatively short and is essentially a mnemotechnical aid for the student , to be learned by heart as a first step in coming to grips with a particular corpus: the layers of commentary then provide exegesis and systematization, as well as extended discussion of controversial questions.
All four schools of Tibetan Buddhism generally follow a similar curriculum, using the same Indian root texts and commentaries.