This is the accumulation of the monthly benefits you should have been paid if your Application was approved. For example, you filed your Application in January 2016 and it was denied. You then filed for Reconsideration and were again denied. You file a Request for Hearing but the hearing date is scheduled for March of 2018. The period between January 2016 and March 2018 constitutes your past-due benefits. It is from the past-due benefits that your attorney will be paid. The attorney will not be paid from your ongoing monthly checks after March 2018 in this example.