General Actions:
Passback is an action that ensures call return of primary adserver´s ad position code from external system (e.g. retargeting, RTB systems,...). It happens in a moment when external system doesn´t have anything to be displayed for the certain impression (e.g. campaigns don´t reach required price, a user doesn´t correspond to campaign´s requests such as geotargeting, age,...)
Read more about passback code usage and setting in iBB AdServer...