The string keeps the ball from flying off.
What is the force exerted by the string called?
It is called centripetal force.
What then is centrifugal force?
It is the force tending to fly off from the centre.
What is centripetal force?
It is the force tending to draw towards the centre.
Why does mud fly off from the wheel of a wagon when in motion?
Because the motion of the wheel gives centrifugal force to the mud, and causes it to fly off.
What does the rim of a wheel when in motion, tend to do?