If the employer plans to reject you due to information obtained on a background check, the employer is required by law to inform you via a pre-adverse action notice. This notice should include a copy of the background check and a copy of "A Summary of Your Rights Under the Fair Credit Reporting Act."
At this point, you will have a certain amount of time (usually five business days) to review the check and dispute any findings. If you do dispute any findings, the employer and the background check vendor that performed the background check must reinvestigate all the disputed information in a timely fashion. If there are errors/incorrect information, the check itself must be corrected, and a new copy must be given to you. Only after the employer has taken these steps can the employer legally send you (the job applicant) a final adverse action notice.