How does Lisp let you redefine the language itself?

Question

I\'ve heard that Lisp lets you redefine the language itself, and I have tried to research it, but there is no clear explanation anywhere. Does anyone have a simple example?<

Answer 1

A reference to 'structure and interpretation of computer programs' chapter 4-5 is what I was missing from the answers (link).

These chapters guide you in building a Lisp evaluator in Lisp. I like the read because not only does it show how to redefine Lisp in a new evaluator, but also let you learn about the specifications of Lisp programming language.