The fighter doesn't exactly absorb the AA defenses as others have said, because doing so would result in the fighter getting shot down or at least severely damaged. Instead it flies an evasive pattern through the enemy AA zones in the path you plan to send your bomber, thus when the AA fires it's defensive shots for the turn most of them miss due to the fighter's higher maneuverability. It may take several fighters to "clear" the path for the bombers.
One thing to keep in mind though, if attacking a city, the city itself can still fire at the bombers you send. The air sweep is only for getting units, not cities, to use up their AA shots.