Scheme language is the scripting language of VRR . Also some small parts of VRR are written in Scheme, mostly initialization routines and handy shortcuts, but also save/load mechanism.
Scheme is interpreted using Guile library, which is a wide-spread Scheme library designed to be used as extension (scripting) language in other applications.
There are several reasons for using Scheme as a scripting language for a program like VRR : Scheme is a dialect of Lisp featuring simplicity and clean design, which makes it pretty easy to learn. Scheme is also often used in computer science curricula. Scheme has standard way to express structured data and integrated parser and writer of that form.
To learn Scheme see the Structure and Interpretation of Computer Programs book (available online at http://mitpress.mit.edu/sicp/full-text/book/book.html). The language standard can be found at http://www.schemers.org/Documents/Standards/R5RS/HTML/.
For more about Scheme cooperation with VRR see Scheme.