LISP: A dynamic car

I have a complex list of lists and want something to check the second element of every list of the list in the loop, any supported features like this?

the dynamic car keeps on truckin next stop, dub central

also can anyone here explain to me what defsetf does and the proper way to do it. I'm reading an Othello program and i see

(defun bref (board square) (aref board square))
  (defsetf bref (board square) (val)
  `(setf (aref ,board ,square) ,val))

and no matter how long i look at this or examine the function that's calling it i can't make out it's intended purpose

also the link to the othello program if you want to see the calls or whatever
http://norvig.com/paip/othello.lisp