You have cup-and-cone bearings where the "cup" is the race built into the hub and the "cone" is a shaped nut that threads onto the axle and forms the inner race. The bearing balls are either loose or held in a thin metal retainer and are sandwiched between the cup and the cone. You adjust the bearings by tightening or loosening the cone and it's held in place by a separate lock nut.
Google "cup-and-cone bicycle hubs" and you will find a ton of articles and videos showing how to service them.