CDR function returns cdr part of cell in the argument, that is list of all elements but first. CDR is identical to REST.

(cdr '(1 2 3)) => (2 3)
(cdr (cons 'a 'b)) => B
(cdr (cons '(1 2 3) '(a b c))) => (A B C)
(cdr '()) => NIL
(cdr nil) => NIL