When an object moves in a circle, its velocity changes. This means it has an acceleration. This acceleration is directed toward the center of the circle and is called the centripetal acceleration. There must be a centripetal force to produce this centripetal acceleration.
The net force on an object moving in a circle provides this centripetal force and is directed toward the center of the circle.
( Think of the icy roads in the winter. With ice on the roads there may not be a sufficient net force available to provide this centripetal force so a car may not be able to make a curve and may go off he road. )
(C) 2003, Doug Davis; all rights reserved