You are reading about SXEmacs which is a self-documenting, customizable, extensible real-time display editor.
SXEmacs is a display editor because normally the text being edited is visible on the screen and is updated automatically as you type. See Display in SXEmacs User’s Manual.
It is a real-time editor because the display is updated very frequently, usually after each character or pair of characters you type. This minimizes the amount of information you must keep in your head as you edit. See Basic Editing in SXEmacs User’s Manual.
It is advanced because it provides facilities that go beyond simple insertion and deletion: filling of text; automatic indentation of programs; viewing two or more files at once; and dealing in terms of characters, words, lines, sentences, paragraphs, and pages, as well as expressions and comments in several different programming languages. It is much easier to type one command meaning “go to the end of the paragraph” than to find that spot with simple cursor keys.
Self-documenting means that at any time you can type a special character, Control-h, to find out what your options are. You can also use C-h to find out what a command does, or to find all the commands relevant to a topic. See Help in SXEmacs User’s Manual.
Customizable means you can change the definitions of Emacs commands. For example, if you use a programming language in which comments start with ‘<**’ and end with ‘**>’, you can tell the Emacs comment manipulation commands to use those strings (see Comments in SXEmacs User’s Manual). Another sort of customization is rearrangement of the command set. For example, you can set up the four basic cursor motion commands (up, down, left and right) on keys in a diamond pattern on the keyboard if you prefer. See Customization in SXEmacs User’s Manual.
Extensible means you can go beyond simple customization and write entirely new commands, programs in the Lisp language to be run by Emacs’s own Lisp interpreter. Emacs is an “on-line extensible” system: it is divided into many functions that call each other. You can redefine any function in the middle of an editing session and replace any part of Emacs without making a separate copy of all of Emacs. Most of the editing commands of Emacs are written in Lisp; the few exceptions could have been written in Lisp but are written in C for efficiency. Only a programmer can write an extension to Emacs, but anybody can use it afterward.