A ball-joint is a crucial part of the suspension of most cars. It’s a coupling that links two bits of the suspension but allows movement between those two in a variety of directions.
Without this ability, the suspension wouldn’t be able to move freely and couldn’t offer the movement that the wheels require to stay firmly on the ground. Suspension doesn’t just go up and down; it needs to move in many planes to cope with everything from potholes to the stresses applied when you turn a tight corner.
As the name suggests, the ball joint is a ball held inside a cup, allowing that orbital movement that’s required. Think about your human hip joints. They consist of a ball (attached to your femur) moving inside a socket or cup (your pelvis). And cars get arthritic, too: when the ball or the cup wears, there’s excess movement and the whole ball joint needs replacing.
Now read about how suspension bushes work.