Spokes are the thin metal rods that radiate from the hub of the wheel.
When a wheel is true and round all the spokes have equal tension. When a wheel starts to wobble it means that there is uneven tension among the spokes. A blow to the wheel, running over a pothole or hitting a curb, for example, can cause uneven tension. On occasion the tension is so great on a spoke that it actually breaks and needs to be replaced. Tension this great is caused, in large part, by the additional weight that we are carrying on our bikes and pulling in our B.O.B.s.