全句是 medieval agriculture in northern Europe fed,clothed and sheltered a predominantly rural society with a much lower population density than it is today.
这里 fed 是 feed 的过去时形式,指喂养,提供食物
全句：北欧中世纪农业为比如今人口密度小得多的主要农村社会提供了吃、穿和住房.