PUSH macro modifies variable or generally place. It makes a new cons cell filled with item as car and previous value as cdr, that is effectively prepends new item to list found at the place. See also PUSH-NEW, ACONS and POP.

(let ((x 'x)) (push 4 x) x) => (4 . X)
(let ((x '(3 2 1))) (push 4 x) x) => (4 3 2 1)
(let ((x '((a b c) (3 2 1) (e f g)))) (push 4 (second x)) x) => ((A B C) (4 3 2 1) (E F G))