GNU Info

(emacs-lisp-intro.info)fill-column Example

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`fill-column', an Example Variable
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   The variable `fill-column' illustrates a symbol with a value
attached to it: in every GNU Emacs buffer, this symbol is set to some
value, usually 72 or 70, but sometimes to some other value.  To find the
value of this symbol, evaluate it by itself.  If you are reading this in
Info inside of GNU Emacs, you can do this by putting the cursor after
the symbol and typing `C-x C-e':

     fill-column

After I typed `C-x C-e', Emacs printed the number 72 in my echo area.
This is the value for which `fill-column' is set for me as I write
this.  It may be different for you in your Info buffer.  Notice that
the value returned as a variable is printed in exactly the same way as
the value returned by a function carrying out its instructions.  From
the point of view of the Lisp interpreter, a value returned is a value
returned.  What kind of expression it came from ceases to matter once
the value is known.

   A symbol can have any value attached to it or, to use the jargon, we
can "bind" the variable to a value: to a number, such as 72; to a
string, `"such as this"'; to a list, such as `(spruce pine oak)'; we
can even bind a variable to a function definition.

   A symbol can be bound to a value in several ways.  Note: Setting the
Value of a Variable, for information about one way to do
this.