FROM THE USCIS BEACON BLOG:

Advance parole is most commonly used when someone has Form I-485, Application to Register Permanent Residence or to Adjust Status, pending. If you depart the United States while your I-485 application is pending without first obtaining advance parole, your case will be denied, unless you fit into a narrow exception for those maintaining certain nonimmigrant statuses. CLICK HERE FOR THE ANSWERS TO FIVE QUESTIONS ABOUT ADVANCE PAROLE.