Access review in SailPoint means to review the access of any identity and take the necessary action. Necessary action means to approve or reject the required approval.
In Certification, When any identity access need to be reviewed in Manager Certification, Application owner Certification, Entitlement owner Certification etc, then access review request is send to particular owner to review that. Manager or Owners can approve or revoke the access.
Access review can be found under My Work -> My Access Review.