ATOM function returns true if the argument is not a cons cell, otherwise it returns false. See CONS and LIST.

(atom nil) => T
(atom 'some-symbol) => T
(atom 3) => T
(atom "moo") => T
(atom (cons 1 2)) => NIL
(atom '(1 . 2)) => NIL
(atom '(1 2 3 4)) => NIL
(atom (list 1 2 3 4)) => NIL