Most of the GNU Emacs text editor is written in the programming language called Emacs Lisp. The code written in this programming language is the software--the sets of instructions--that tell the computer what to do when you give it commands. Emacs is designed so that you can write new code in Emacs Lisp and easily install it as an extension to the editor. This is why Emacs is called the "extensible editor".
(Indeed, since Emacs does so much more than provide editing capabilities, it should perhaps be called an "extensible computing environment", but that phrase is quite a mouthful. Also, everything you do in Emacs--find the Mayan date and phases of the moon, simplify polynomials, debug code, manage files, read letters, write books--all these activities are kinds of editing in the most general sense of the word.)
Although Emacs Lisp is usually thought of in association with the text editor, it is a full computer programming language. You can use it as you would any other programming language.
Perhaps you want to understand programming; perhaps you want to extend Emacs; or perhaps you want to become a programmer. This introduction to Emacs Lisp is designed to get you started: to guide you in learning the fundamentals of programming, and more importantly, to show you how you can teach yourself to go further.
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