Pillow ball suspension (or pivot ball) is a type of suspension that is common on some types of RC cars, usually ones at larger scale (1/8th), but occasionally found on 1/10-scale cars as well.
With pillow ball suspension, the suspension arms are fairly short, and a ball is threaded into the end of the arm, and that ball is then held by the hub (upright) which in turn holds the axle.
The other common type of suspension pivot uses a pin that is held by the arm on which the hub is attached and is able to rotate. In the front of a car, this is known usually as C-hub suspension.
Pillow ball suspension allows you to easily change the kingpin inclination (and therefore scrub radius) of the car, but with c-hub suspension these changes require changing the c-hub itself. Pillow ball suspension also allows you to change the track width of a car easily.