FIRST function returns the first element of list, that is the car part of its cons cell argument. FIRST is identical to CAR.

(first '(1 2 3)) => 1
(first (cons 'a 'b)) => A
(first (cons '(1 2 3) '(a b c))) => (1 2 3)
(first '()) => NIL
(first nil) => NIL