can you explain the sentence about defvar in elisp?

[waterloo]

When you specified a variable using the `defvar' special form, you
could distinguish a readily settable variable from others by typing an
asterisk, `*', in the first column of its documentation string.  For
example:

     (defvar shell-command-default-error-buffer nil
       "*Buffer name for `shell-command' ... error output.
     ... ")

I can not understant it "distinguish a readily settable variable from others" ?

Thanks