Newton's Second Law of Motion

If an object accelerates, then it must have a net force on it.

Sometimes it is clear what that force is. But sometimes, it may not be.

Consider an object moving in a circle.

When an object moves in a circle, its velocity changes. Even with uniform speed, the velocity changes because the direction changes. This means it has an acceleration. This acceleration is directed toward the center of the circle and is called the centripetal acceleration.

This means there must be a centripetal force -- directed toward the center of the circle --to produce this centripetal acceleration.