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.

  (defun main
    ()
    (format t "Hello world")
  )

Should be

  (defun main ()
    (format t "Hello world"))

The lisp community has very well established formatting conventions. You pretty much never see closing parens on their own line like this except when reading code written by beginners (or some special cases like package export lists for example, where one might want to leave the list "open" for adding new symbols, similar to a trailing comma in some other languages).