In German we call it "Wellenbrecher", which means as much as breakwater. It makes sure nobody gets crushed when the whole crowd is moving, so it basically breaks this wave of pushing. To get in the first zoned area you have to be there early enough, no special tickets needed. When there are too many people, security closes the first zone and you have to watch the concert from behind those fences. Once you're in the first area you also can't get out and come back again easily. Keep that in mind if you have an overactive bladder..