Dangling parens are the loose closing parens on separate lines. I.e. this:

  (defun plus (x y)
    (+ x y)
  )

instead of this:

  (defun plus (x y)
    (+ x y))

It suggests the author tires to balance parens manually/visually rather than using a proper editor mode. An understandable and common mistake of a newbie, but looks bad when you are writing a tutorial.