Complex sentence: The dialect used in GNU Emacs is known as Emacs Lisp (Elisp), layered on a relatively small and stable core of basic services and platform abstraction written in the C programming language.
Split into simpler sentences: The dialect used in GNU Emacs is known as Emacs Lisp (Elisp). The Lisp layer sits atop a stable core of basic services and platform abstraction written in the C programming language.