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(emacs-lisp-intro.info)Args as Variable or List

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An Argument as the Value of a Variable or List
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   An argument can be a symbol that returns a value when it is
evaluated.  For example, when the symbol `fill-column' by itself is
evaluated, it returns a number.  This number can be used in an addition.

   Position the cursor after the following expression and type `C-x
C-e':

     (+ 2 fill-column)

The value will be a number two more than what you get by evaluating
`fill-column' alone.  For me, this is 74, because the value of
`fill-column' is 72.

   As we have just seen, an argument can be a symbol that returns a
value when evaluated.  In addition, an argument can be a list that
returns a value when it is evaluated.  For example, in the following
expression, the arguments to the function `concat' are the strings
`"The "' and `" red foxes."' and the list `(number-to-string (+ 2
fill-column))'.

     (concat "The " (number-to-string (+ 2 fill-column)) " red foxes.")

If you evaluate this expression--and if, as with my Emacs,
`fill-column' evaluates to 72--`"The 74 red foxes."' will appear in the
echo area.  (Note that you must put spaces after the word `The' and
before the word `red' so they will appear in the final string.  The
function `number-to-string' converts the integer that the addition
function returns to a string.  `number-to-string' is also known as
`int-to-string'.)