Hello, I need help understanding the following problem in LISP

The solution has me confused, please help me understand , thank you for your help in advance

5) Define function f5 that accepts a nested list of elements and returns the list with the elements which are integers. Assume that integerp is already a function available to you which returns "T" if its argument is an integer, otherwise, it returns "NIL". For example LISP> (fs (7 (a 3) 9 b)) (7 (3) 9) (defun f5 (x) (if null x) (if (atom (car x) (if integerp (car x)) (cons (car x) (fS (cdr x))) (fs (cdr x)) (cons (f5 (car x)) (f5 (cdr x))