Scheme is a programming language in the Lisp family, it is used internally by the higher level language LilyPond — for music engraving.

LilyPond uses the Scheme programming language, both as part of the input syntax, and as internal mechanism to glue modules of the program together.

— from the LilyPond documentation - Scheme tutorial.

Scheme is a functional programming language. It is designed to provide a mathematically well-founded language based on lambda calculus (with eager evaluation).

It follows a minimalist design philosophy specifying a small standard core with powerful tools for language extension. Its compactness and elegance have made it popular with educators, language designers, programmers, implementors, and hobbyists.

The Scheme language belongs to the lisp Family. Since Scheme uses a single namespace for naming functions and other values, it is called a lisp-1.

— from Stack Overflow's Tag Info page.

Question on Scheme that are not related to LilyPond are not on-topic for SE: MP&T. These questions should be asked on Stack Overflow.