The bobber is the bait angler's strike indicator. It floats on the surface with a hook and bait dangling below; when a fish takes, the bobber pulls under, telling you to set.
Two main styles: fixed bobbers clip to the line at one spot (great for shallow water, ideal for kids learning), and slip bobbers thread onto the line and slide up to a stopper, letting you fish deeper while keeping the rig castable. Slip bobbers are the more versatile of the two for serious bait fishing.