pam_rootok is a PAM module that authenticates the user if their UID is 0. Applications that are created setuid-root generally retain the UID of the user but run with the authority of an enhanced effective-UID. It is the real UID that is checked.
In the case of the su(1) application the historical usage is to
permit the superuser to adopt the identity of a lesser user
without the use of a password. To obtain this behavior with PAM
the following pair of lines are needed for the corresponding entry
/etc/pam.d/su configuration file:
# su authentication. Root is granted access by default. auth sufficient pam_rootok.so auth required pam_unix.so