Question

What is meant by ball bearing? How are they useful?

Answer 1

A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races. The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads.

These r used in fidget spinners...

Answer 2

Ball bearings are also known as roller bearings.
Ball bearings helps the substance to move without facing much friction because these ball bearings rotates and rolling friction is less than sliding friction.