When a body moves in a circle, its direction is constantly changing. Since velocity is a vector quantity, having both a size and a direction, the velocity of the body must be changing even though its speed is constant.
As the velocity of a body in circular motion is changing, it must also be accelerating. The force responsible for this acceleration is called the centripetal force and it acts radially inwards towards the centre of the circle.
For example, a car travelling on a roundabout will move at a constant speed, but with a changing velocity, as its direction is constantly changing. The centripetal force that acts inwards is due to the friction between the car's tyres and the road. This force keeps the car moving in a circular path.